I had this issue a while ago too but it was a hardware problem which was then repaired by nokia. So I certainly think that these problems are in no way software related.

If it is a hardware issue, my suggestion is a piece of tape on the back of your sim. Trim it to the shape of your card. Do not fold excess over the edges as that will bend out the cage.
Depending on the type of tape (i used electrical)...it should give you enough pressure to keep your simcard pressed to the contacts.

I may have bent my sim removing it from my old Palm...so i had to use this solution. But for a while i could disconnect from the cell network just by turning my N900 upside down.

i have the same issue and have done a complete reflash....but still keep getting intermittent problems with the sim....never had this problem until pr1.3

Reflashed 1.3 (including latest eMMC), but still get problems. Will try to revert to 1.2. Should this pose any other problems?

Nevermind, just went ahead and reverted. Didn't help at all. I think perhaps this phone needs to be sent to service...

I've been having this same problem for the last couple of weeks.

Have re-flashed, and still the problem exists so I'm convinced the problem is either the N900 hardware or the SIM card.

My question: Has anyone else with the problem fixed it by getting their operator to replace the SIM card?

Or should I just bite the bullet and get it send back to Nokia (~shudder~)?

I sent it to Nokia. It wasn't the SIM card - it worked fine in other phones, and other SIM cards didn't work in the N900. Figured it had to be a hardware fault.

Just got the repair report. Seems like it was an electrical failure, i.e. HW error, not SW. Also says "phone light swapped", don't know if it has anything to do with it.

One more here! But I am using a dual SIM adapter, I thought it could be it...I guess not now.
But I am pretty sure it is software related, either the pr1.3 or these new kernels or something else, but not hardware related, how come it was working perfectly before the update?

Has anyone tried unistalling the custom kernel to see if it stopped?

Damn...sometimes it sucks...what I do is...go to offline mode for a while and when I go back to normal mode the signal is back.

We now have 4-5 threads about what well could be the same, or at least very related, problem. It is bad for traction. "communication problem sim"