battery-eye had nothing to do with it, it still happens. I try to send a few SMS to myself, and boom, sim card disconnected. For some seconds.

I had this problem, turned out that my sim card wasn't staying in the slot properly, it was sliding up slightly and therefore not making contact.
Try cleaning and inserting the sim card, then try to slide it up and down with the sim tray shut.
If it moves, add a bit of electrical tape to the sim card just to hold it a bit better.
Or if you can be bothered, get a new sim from your network.

i have this from the beginning that i have the N900
now and then bang. sim card connection lost
Sometimes a reboot helps
sometimes i really have to take out the battery (for a while some times...)

one time 1 couldnt really recover (maybe a full flash would) and i send it back to nokia,
got mine back flashed and working again, but soon after it happened now and then

Now with 1.3 it happened also to me also once more
But now i really can get it happening all over the place and i cant recover what ever i do.. what i do then? Install the power kernel. Then its a nogo. from boot on it will not find my sim card. Only reverting to stock kernel will do it.
Weird thing is that i have had the power kernel all the time with 1.2.. (and yes then it also did happen now and then but thats no different then not using the power kernel)

ive never noticed this problem, im still on FW1.2

James_Littler: I've tried that several times.

I had this happen once to me in pr1.2, and its happened once again already in pr1.3.

I also updated OTA, but am going to reflash with both .bin files and see what happens. I'll report back if I still have issues with this.

Oh, and sometimes it happens even when I simply open the SMS application (or an individual conversation).

It happens for me when I press on the back of the phone, or for example press hard on h on the keyboard. Also whenever I have the phone in my pocket and run or walk faster. Easily noticeable with streaming audio over 3g, when it always stops when I run, and then works perfectly when I walk slowly in the same area.

I sent it to nokia for repairs, it was gone 2 weeks and a few days, and it's behaving exactly the same after.

I'm convinced this is a hardware issue. I had it and sent it off to Nokia for repair, they sent a brand new replacement.

Doesn't seem to react to pressure/movement, I'll keep it in mind though.