Wow, I have been living with this same exact crap for months and I always thought it was just my phone. I seem to get a few good weeks then it's back, plus mine will sometimes show no signal (2G or 3G) for up to a few hours. I notice if I try to go into 'Phone' under 'Settings' when it's doing this it hangs and doesn't want to read the SIM. I'm losing track of how many times I've flashed my N900.

Took my phone to my local t-mobile store last week and got my SIM swapped with no difference. Interesting part was that they put it in an HD2 it did the same thing! Submitted a trouble ticket to t-mobile engineering and over the weekend received an "issue has been resolved" message.

At the moment I cannot get continuous throughput on 3G even though it shows full bars, it just stops downloading. Also, if I change the APN and try to reconnect to the internet it will often fail initially. Phone calls over 3G appear to be working for now.

Questions for others with the same problem:
-Have you tried your SIM in another phone and if so what did it do?
-For those who have sent their phone back to Nokia, is it working now? Perhaps they did more than just the standard reflash?

Thanks for reading and good luck.