More easy way, and it works if you don't have SD card.
Important: It's for overclocked phones, if you dont have kernel-power installed then wait someone make this for stock kernels.

1. If you have uboot or bootmenu, remove it.
2. Install rootsh and leafpad
3. Download NITDroid rootfs from here.
4. Download NITDroid kernel from here.
5. Copy downloaded files to MyDocs.

6. Open x-term and do this:
sudo gainroot
mkdir /and
mount /home /and
apt-get install multiboot multiboot-kernel-power bzip2
cd /home/user/MyDocs
bzip2 -d NitDroid-0.0.8-Popov.tar.bz2
cd /and
tar xvf /home/user/MyDocs/NitDroid-0.0.8-Popov.tar
dpkg -i /home/user/MyDocs/nitdroid-kernel-2.6.28-06_final1_armel.deb
umount /and

7. Make the multiboot item as ammyt says in first post.
8. Apply FIXES postes in first post if you want

That's all. You don't need any SD card!