I recently did an OTA update to PR1.2 and got similar battery problems (dead in 2-3 hours). In my case, Conky showed me that the CPU clock-rate never goes below 500MHz (even when only at 5%-10% CPU load). I don't have the overclocking kernel installed, so that is not the root of the problem. But, as others have suggested, the easiest way to solve a problem like this is simply to reflash:

1) Run the backup-app and do an internal backup in N900
2) Plug your N900 into a computer and copy all files to your PC (the reflash will delete EVERYTHING, including photos, music, etc)
3) Reflash everything, both standard flash and eMMC flash. Howto at:
http://wiki.maemo.org/Updating_the_tablet_firmware
4) Copy all your files back to your N900 (make sure you place the backup directory where it was before)
5) Run the backup-app again to restore everything.