I had the same problem. While I was abroad for three months I used mine as an alarm clock, mp3 player, surf the net and make Skype calls via free WiFi. (N900 is a brilliant traveling companion!) All that time my SIM card was safely tucked in my wallet.

Right after I returned to Helsinki I reinserted my SIM card and got that same error message. I even bought a prepaid SIM card but it made no difference. After getting back home, doing a "hard reset" and still not being able to make or receive calls, I finally gave up and sent my phone to be fixed.

Today -after three long weeks- I finally got a new replacement. I don't know what was wrong with the old phone, but obviously it was easier for Nokia to give a new one than to fix it.

She's now feeding (ie. battery is being charged) and soon I can start to install applications and stuff. I just hope that the latest PR is already in there. Three weeks with a really dumb phone was really hard after being used to n900! *shivers*