Be cautious in reverting to past versions of Maemo 5 firmware. As I understand it, some updates include new firmware for the cellular chip in the N900. Nokia has warned that reverting to older firmware will not restore the older version of the cellular chip firmware -and- the older N900 firmware won't be able to talk to the newer cellular chip firmware. According to Nokia, you'll be left with a phone with no cellular capability.

The early testers of 51-1 were told not to flash back to previous firmware for this reason.