My first N900 is now in for warranty repair for this issue. The dmesg log was full of problems with the modem going away or resetting itself. It could be desoldered simcard slot, but I think the crossed out sim icon is just a generic symbol for communication issues with the modem.

Bought a second N900 because last time I gave it to Nokia for warranty repair it took 2 weeks and I was stuck using a symbian phone. It was a very traumatizing experience, and on top of it all they didn't really fix the problem with my n900. It eventually degraded so badly that now I'm hoping even the repair people will notice it..