I've had this error three or four times over the two years that I've had my N900, and on all occasions, it's been fixed without changing either the phone or the SIM.

For me, the problem happens when I'm trying to do too much in the way of overclocking / undervolting. I didn't make the connection the first time and reflashed the whole phone. Some months later, it happened again whilst I was playing with higher CPU speeds; I went back to slightly more conservative settings (250-900) and the problem went away.

Recently (many months after the above, same hardware), I started playing with 250-1100, with SmartReflex on VDD2 only... it seemed stable. But, a few hours of that, and some use of the memory card, the SIM dropped out. I reverted to my previous settings, and it reverted to working flawlessly.

I haven't isolated the problem further yet; perhaps I could sustain 1100MHz with more voltage and no SmartReflex, but I'm not sure that I intend to try. The observed evidence from two years with the same device is that my particular N900 is happy and stable at 250-900 with SmartReflex on VDD1 and VDD2, and that pushing it further than that works for a few hours, but is not stable.

Your mileage may vary; some people will have a loose or malunctioning SIM card, an unlucky few will have a broken phone, but I've reproduced the problem enough to be fairly sure that in my case, this is the cause.