I have finally managed to recover my N900. Story follows:

1: Charged the device fully.
2: Started the 1.1 over-the-air upgrade
3: Looked fine. Until I noticed that wi-fi didn't work.
4: apt-get upgrade -s indicated that it had only upgraded half-through.
5: application manager still showed Maemo5 as upgrade. Selected it again.
6: After 5: application manager still showed Maemo5, and 21KB left to install.
7: N900 started showing UIBFS errors in 'dmesg' in terminal.
8: It suddenly died and would not power on.
9: Attempted reflash. No go.

At this point, and after hints from posters above, it turned out that the N900 battery was actually discharged (even though it had been fully charged before attempting the initial upgrade). I verified this by measuring the battery with a multimeter.

10: Bought stand-alone charger, recharged battery, checked with multimeter.

I still could not reflash, even with fresh battery:

* Pressing 'u' then connecting N900 to Linux computer did not result in anything (it did not go into reflash mode)
* Connecting, then re-inserting battery in N900 did not work either
* Checking DMESG output on Linux computer while connecting N900 showed no awareness on the USB bus at all
* Removing and re-inserting battery on N900 resulted in a 0.2 second cycle of USB connect, 'Nokia USB ROM' message (or some such), USB disconnect cycle. There was an USB event on the PC _only_ if re-inserting the battery, never if just connecting USB cable (with or without 'u' key).

I finally managed to fix everything by 'cold-flashing' the N900. This is what worked:

1: Make sure that 'usbfs' is mounted on /proc/bus/usb on Linux laptop
2: N900 battery fully charged, as indicated above
3: Remove battery from N900
4: Connect USB cable to Linux computer and N900
5: Execute the following on Linux computer (as root, or with sudo)
6: Re-insert battery in N900
7: Voila! N900 reflashes, then reboots.

Note that I did attempt step 5 on a desktop Linux computer with USB 2.0, but could not get it to work. Presumably the flasher didn't manage to detect the exceedingly short USB connect event when the battery was inserted. I finally got it working with an (old) laptop with USB 1.1.

This was hairy stuff. Apparently boot loader was destroyed on the N900, or something else that made the normal (warm) reflashing fail. NB: Note that "-S usb" is needed with -c, and -h needs "RX-51:2201". The wiki doesn't mention "-S usb" IIRC, and without it it'll try to cold-flash over the serial device instead. The wiki also says the device id (2201) should be visible under the battery, that is not the case.

Things would have been easier if the N900 could run on AC alone, like the Pandora.

Finally I received the external battery charger... and it worked! I was able to flash my N900 but I had to use my Windows XP netbook (win 7 64-bit didn't seem to want to detect the device).

However, I can't get this thing to detect my micro-sd card...

EDIT: Nevermind, it seems to have been detected after waiting a couple of minutes.

Mine worked too!

As I had mentioned before, mine was stuck in the cycle off attempting to charge for a few seconds, boot, run out of power and repeat.

I got my charger a few weeks ago, let it charge all day and then used a windows xp (instead of my Windows 7 - 64bit / beta) to flash with the flasher 3.5 software (instead of the software updater).

After I put the battery in I fallowed the steps and it worked flawlessly. Even my old data was still there.

I wonder, is this some kind of bug that the phone can get stuck in this cycle? I would expect that I should have been able to get around this by just plugging it into the standard wall charger. Or is this just what happens when the firmware flash gets messed up?

When the flash gets messed up, as you say, then charging won't work. That's the catch-22 that forced some of us to buy a desktop charger.

TA-t3 I'm glad you got your n900 up and running again.

You must of absolutely sh1t yourself when you bricked it!

I'm really weary of updating to Pr1.1 specifically the wi-fi issues some users have had.

How is you n900 behaving itself since the update?
Have you encountered any other problems that other users have experienced?

My N900 behaved perfectly fine on 1.0, and 1.0.1. The 1.1 upgrade left the device in a state needing reflash. After reflash it has behaved perfectly again - I don't suffer from reboot loops, battery problems, or anything else. If anyone have problems with e.g. wi-fi after the upgrade I recommend flashing the device right away, while the device can still be reflashed the "normal" way.

So, avoid over-the-air flash and do local usb cold-flash instead?

(I'm just paranoid about wi-fi!)

OTA works for some, not for others. I haven't found a pattern to it. My system failed and I couldn't see why - there was no apparent reason for not fully upgrading/installing OTA.

I'd recommend going with OTA first. If you have issues then reflash using flasher. I'd recommend against using NSU at all - it seems to cause a lot of the flashing issues.

Honestly, I am fed up! Tried everything, but nothing works. My N900 just won't go into flashing mode. IT JUST WON'T!!! I've never had any problems with my other Nokia cellphones, and this experience has left a bad taste. I've given the device at Nokia Care Centre. Once it is in it's original state, i'll probably just sell it off. I am using a 32- bit windows. The binary and flasher work fine, but the device won't go into usb mode due to which the flasher does not detect it. Just some bloody looping keeps happening. Fully charged battery too did not help. Super frustrated!