I have that problem, too.

I have a sim card (new one!) by O2 Germany - it worked fine the first two days, and now it seems that it is not recognized:
- The N900 asks for the PIN at startup
- The phone does not connect to O2
(- Offline mode is diabled, of course)

When I put in an T-Mobile Germany sim card (an older one which does not support 3G), 2G connections seem to work.

So, is this a firmware issue? And when does the new firmware come out?