If your device ignores pressing the u button and looks like it is trying to boot (best way to tell is the screen lights up.. if the u key works, it should stay unlit) then chances are your battery is flat..

I know you've spoken about charging, but a faulty n900 (one that doesn't boot or run right) will often not charge at all, no matter what signs it is giving you

find someone with another N900, 5800. X6 or 5230 handset and use theirs to charge your battery..

you can't flash until the battery is full..