FIXED
I also tried sticking it into Windows 7 to flash it, and it didn't work. (came up with "device disconnected" after I plugged it in while holding u)

This is what I did:
Install the flasher:
http://wiki.maemo.org/Documentation/...mo_Flasher-3.5

Download the kernel into the program files/maemo/flasher 3.5 (or whatever files is there:
http://tablets-dev.nokia.com/nokia_N900.php

Then, plug in your device. if you get my error during driver installation, get this, and install it, and plug in the device before running it. it will create an alternate driver for windows to use:
http://sourceforge.net/projects/libu...0.zip/download

once you've done that, open the flasher, and run the following command (replace the bit in bold with your version of the maemo):
flasher-3.5 -F RX-51_2009SE_20.2010.36-2_PR_COMBINED_MR0_ARM.bin --flash-only=kernel -f -R

that's it! you should be able to go in to maemo after pressing 0!

hope that helps people.