Somebody will correct me if I'm wrong, but I remember there was a bug relating to the sim card becoming unrecognized. The one thing you haven't tried is flashing it to the latest firmware. If you are already running PR1.1.1, then try reflashing it.

If that STILL doesn't help, pull all your data off the N900 (pictures, music, movies, docs, etc), and flash both the storage and firmware.

If none of these work, you may have a hardware problem.