Hello, first sorry for my english, i live in Peru.

I bought a couple N900 to sell here (there is no Nokia store in Peru) and one of them had exactly the same problem.

After updating one of them, I had the same error and my WinXP or Ubuntu didnt recognized it. I have to do this:
First make sure that your battery is charged, since the N900 is dead, if you connect the charger or the USB cable it WILL NOT charge.
Remove the battery for about 2 or 3 hours
Connect the USB cable (WITHOUT the battery). An orange Led will be flashed for about a second.
Put the battery in while pressing the U key, wait for a few seconds, hopefully it will vibrate and be recognized again, so you can flash using flasher-3.5

If that doesnt work, try again in a few minutes. I spend a whole day trying to save this N900, because here in Peru, there are no Nokia Care Centers...
But now it is alive

Good luck!