Since a few days i encounter the same problem.
I cleaned the simcard and the slot.
I've put a piece of paper between simcard and battery
I lifted the pins of the simcard slot a bit to make better contact.
I reflashed both firmware and EMMC.
I removed SD card (read somewhere that this could be an issue as well)
I tried a different battery.
Even without a simcard i get this message telling me that there is a communication error. I expected a simcard icon with a red dash through it, but not the error message.

Right after I got the message I don't have an IMEI anymore.
I used to get the message after a while when I restarted my N900. Now I get the message even before the welcome animatiion starts.

Is there anything left to try out before I ship it to a nokia care center?