Long ago we wrote about the Ubuntu 9.10 ported to Nokia N900.
Recent data are as follows: on the Nokia N900 can be installed Ubuntu 12.04 (it will be released in April) and run it with the u-boot.
For details and to discuss possible on the forum.
Nokia N900 and Ubuntu 9.10
Ubuntu Mobile on the Nokia N900
Quick install Fremantle SDK (Maemo 5). Installing Qt 4.6. Running and debugging in the emulator and on device
Mozilla has released the release candidate of Firefox 4 for Maemo (Nokia N900)