@eiraku i've read somewhere someone having trouble flashing cos of the same issue with N900 booting up as soon as the usb plugs in. As far as i can recall you have to unplug the battery aswell. I think this is the steps to get it to flash.
1. hold U
2. plug in usb
3. push battery in with backcover (maybe it'll work without backcover).
This is just on top off my head of what i can recollect of something i read a while back
Try it out if you like i'm pretty sure it couldn't hurt