Putting together instructions from various threads, I now have a quad-booting phone. Why, you may ask? Because I can. And because there are more apps for Android, Meego is under more active development, Maemo is more stable, and Kubuntu just plain looks cool. I decided to post how I did it.

What you will need:
1. N900
2. MicroSD card, at least 4 GB
3. MicroSD card, at least 8 GB class 6 (I used a class 10)
4. A computer that you can reflash from if necessary (i.e. not running Vista or Windows 7); also one that has a card reader (either built-in or external).
5. A lot of patience

Ready? Let's go.
1. Install MeeGo.

Download the Meego Summer Release from here. Follow the instructions on the wiki for installing to the MicroSD card, using your 4GB card. For me the procedure was (under Linux):

Code:

sudo apt-get install pv
sudo umount /dev/sdb1
cd Downloads
bzcat meego-handset-armv7l-n900-1.1-mmcblk0p.raw.bz2 | pv | sudo dd bs=4096 of=/dev/sdb
mkdir meego-modules
sudo mount /dev/sdb2 /mnt
cp -r /mnt meego-modules    #these modules are used later for Kubuntu
sudo umount /mnt

MAKE SURE YOU KNOW WHAT DEVICE YOUR SD CARD IS! Mine was /dev/sdb but yours may not be (especially if you have more than one hard drive).
Now, go to your N900 and install multiboot. Then, open a terminal.

Code:

root
cd /home/user/MyDocs
mkdir uboot
cd uboot
wget "http://repository.maemo.org/extras-devel/pool/fremantle/free/u/uboot-pr13/uboot-pr13_1.0.2_armel.deb" -without quotes
dpkg -X uboot-pr13_1.0.2_armel.deb ex
cd ex/boot
flasher -u -F uboot-and-zImage-2.6.28-20103103+0m5.fiasco
cp zImage /boot/multiboot/vmlinuz-2.6.28-omap2
cd /lib/modules
cp -rf 2.6.28-omap1 2.6.28-omap2
cd /etc/multiboot.d
cp 00-Maemo-2.6.28-omap1.item. 02-Maemo-2.6.28-omap2.item
nano 02-Maemo-2.6.28-omap2.item

#type this in:
ITEM_NAME="Maemo 2.6.20-omap1 with uboot"
ITEM_KERNEL=2.6.28-omap2
ITEM_MODULES=ext3

# type ctrl+x to exit, hit y, and then enter

If you have the power-kernel, also install kernel-power-bootimg.
Shut down your N900, and then power it on with the slide open. If the multiboot menu comes up, hit whichever number has the uboot entry (number 3 for me). Do NOT type "run mmcboot" in uboot, it won't work. Instead, just let it time out and it should boot into Meego. Verify that everything is more or less working, including the phone.

2. MeeGo working? Good. Install Kubuntu Mobile.

Go back to your computer. Launch a partition utility (I used GParted). Format the 8GB microSD card in the following way:
- A 6.2 GB ext3 partition;
- A 1 GB swap partition;
- and a fat32 partition that fills whatever space you have left.
Open a terminal.

Code:

mkdir Liquid;cd Liquid
wget -c http://cdimage.ubuntu.com/kubuntu-mobile/ports/releases/10.10/release/
gunzip maverick-preinstalled-mobile-armel+omap.img.gz | pv | sudo dd bs=4096 if=maverick-preinstalled-mobile-armel+omap.raw of=/dev/sdb1
sudo fsck -cy /dev/sdb1

AGAIN, MAKE SURE /dev/sdb1 IS YOUR MICROSD CARD.
If fsck gives errors, the image may be corrupt; try starting again from the wget line.

Code:

sudo apt-get update
sudo apt-get install qemu-arm-static
sudo cp /usr/bin/qemu-arm-static /media/a6c5c4e9-555c-4565-ba1d-ceea8d5a953f/usr/bin/
sync
bzip2 -d ../meego-handset-armv7l-n900-1.1-mmcblk0p.raw.bz2
sudo mount -o loop,offset=512 -t auto meego-handset-armv7l-n900-1.1-mmcblk0p.raw /mnt/
sudo cp -a /mnt/lib/modules/2.6.35.3-13.10-n900/ /media/a6c5c4e9-555c-4565-ba1d-ceea8d5a953f/lib/modules/ -a
sudo cp /mnt/lib/firmware/* /media/a6c5c4e9-555c-4565-ba1d-ceea8d5a953f/lib/firmware/
cp ../meego-modules/* /media/B68E-53AB
sudo umount /mnt
sudo chroot /media/a6c5c4e9-555c-4565-ba1d-ceea8d5a953f/
echo "nameserver 8.8.8.8" >> /etc/resolv.conf
exit
mkdir meego;cd meego
git clone git://gitorious.org/meego-device-adaptation/n900_nokia-n900-configs.git
git clone git://gitorious.org/meego-device-adaptation/n900_udev-rules-nokia-n900.git
cd ..
wget -c http://ianlawrence.info/downloads/liquid/essential-kubuntu-mobile.sh
chmod +x essential-kubuntu-mobile.sh
nano essential-kubuntu-mobile.sh

Edit the following lines to match your directory structure:

Code:

MPATH="/media/a6c5c4e9-555c-4565-ba1d-ceea8d5a953f"
GITPATH="/media/80GB/Liquid/meego/"

This is what I had:

Code:

MPATH="/media/a6c5c4e9-555c-4565-ba1d-ceea8d5a953f"
GITPATH="/home/skykooler/Downloads/Liquid/meego/"
USER="skykooler"

Now run the script:

Code:

sudo ./essential-kubuntu-mobile.sh

Eject the MicroSD card, put it in your N900, shut it down, and start it up again. Kubuntu will take a little while to boot. When it does, you should see a login screen. Enter your login information, and tap the login button (the right-pointing arrow). Now wait for a while, the login took mine nearly 5 minutes. If you are patient you should be rewarded with a working Kubuntu!

3. Ok. Now for NITDroid. This part is more dangerous but worth it. We are installing on eMMC. This should not wipe your data if you follow the instructions, but it could happen, so back everything up.

To boot Maemo, type "run noloboot" in uboot. Go to this page on your N900. Download the following files to your base folder (MyDocs):
http://downloads.nitdroid.com/e-yes/N12_UMay.tar.bz2
http://downloads.nitdroid.com/e-yes/..._rc4_armel.deb
http://ftp.de.debian.org/debian/pool....1-1_armel.deb
http://forum.xda-developers.com/atta...4&d=1310073004
Connect your N900 to your computer in Mass Storage mode. If it says "device storage in use", reboot it and try again. Then copy ALL files from it into a folder on your computer. IF YOU DON'T DO THIS YOU WILL LOSE ALL OF YOUR DATA.
Install Fdisk on your N900 with the following commands:

Code:

root
mkdir -p /home/user/util-linux /home/user/bin
cp /home/user/MyDocs/util-linux_2.13.1.1-1_armel.deb /home/user/util-linux
cd /home/user/util-linux
dpkg-deb -x util-linux_2.13.1.1-1_armel.deb root
cp root/sbin/fdisk /home/user/bin/
cd ..
rm -rf util-linux
umount /home/user/MyDocs
/home/user/bin/fdisk /dev/mmcblk0

Delete partition 1 (./MyDocs):

Code:

d
1

Create new re-sized primary partition:

Code:

n
p
1
First cylinder: 2
Last cylinder: 800000

Create new extended partition:

Code:

n
e
First cylinder: 800001
Last cylinder: 884865
n
First cylinder: 800001
Last cylinder: 884865
t
1
76
t
5
76

Point of no return - writing the table and syncing :

Code:

w
reboot

When it has started up again, execute these:

Code:

root
/home/user/bin/fdisk /dev/mmcblk0
t
1
0c
w
mkfs.vfat -F 32 /dev/mmcblk0p1
mke2fs -L NITDroid -j -m0 /dev/mmcblk0p5
reboot

Connect your N900 to your computer again and copy all the files you backed up back on to it. Disconnect it, and run (still on the N900):

Code:

root
mkdir -p /and
mount -o noatime,rw /dev/mmcblk0p5 /and
cd /home/user/MyDocs
rm -rf /and/*
bzcat N12_UMay.tar.bz2 | tar xvC /and
dpkg -i nitdroid-kernel-2.6.28-07_rc4_armel.deb
cp setcpu_2.24-donate.apk /and/system/app/
exit

Now shut down your N900 and power it up again with the slide open. Hit whichever entry says "nitdroid" in it (4 for me). It should boot successfully, but may not. If it doesn't, keep rebooting until it does. When you get to the screen that says "tap the android to begin", tap random places around the screen until it actually does something. Open SetCPU and tap "Autodetect", then grant it superuser permissions. Then set the Max value to 805 and check Set on Boot. The reason for this is that by default max is set to 1150, and most N900s are not stable at this speed, meaning you will get frequent crashes and freezes until you do this.
Note that NITDroid doesn't seem to like to start up. I generally have to boot it about 5 to 10 times for every 1 successful boot. However, once booted it should be plenty stable. 3G, wifi and bluetooth all work; calls to not. I have heard SMS works, but have not tested it as I do not have texting on my service plan.

Always boot Maemo between any boots of MeeGo or Kubuntu and NITDroid; otherwise you may mess up your kernel and need to reflash.
If you do need to reflash, you can do so without destroying your data; flash only the kernel. See http://wiki.maemo.org/Updating_the_t...a_FIASCO_image for instructions.


And pictures: