Man, use Google, seriously.

https://github.com/skry/linux-n900

The N900 can run newer kernels (Skry's patchset IIRC can even be applied to 3.8 series, the newest kernels)

Fremantle on the other hand, is made of old components that break ABI-wise with new internals (a large part of which are open source!)
This is to be remedied soon enough by the Fremantle-on-Squeeze project.