Same problem , started in the end of April.
Was running older Maemo release when problems started.
Upgraded to latest rel but issue remained.

Phone was two weeks in repair and they could not replicate the issue. Phone was flashed there.

Tried different SIM cards but issue still remains.

Today gave phone to alternative repair shop.
Interestingly customer right before me had exactly the same issue with n900.

Only thing i noticed during SIM problem are acwake events in dmesg. Not sure what they stand for.
[42212.271911] McSAAB: ACWAKE DOWN
[42212.271972] McSAAB: ACWAKE UP
[42212.272033] McSAAB: WAKELINES TEST OK

After 2 weeks I got my N900 back from repair. They changed it with a new one. Evidently the only solutions, as some users said, are a new phone or a new PCB board.

I'm planning a send to base as well. This is becoming ridiculous. Today I spent 15 minutes fiddling with it.

Did any of you play for the fix?

Got phone back second time now.
They discovered some moisture damage as usually and just did some chemical cleaning. Phone worked for 1h and then failed again.
Now i probably need just to convince them to change PCB and pay for it.
Grr frustrating , already almost 1month using some crappy alternative phone.

Well, im hoping that my insurance company dont send it back "fixed" as other people have had done.
Its just a waste of time and postage..... Anyway i checked with them already they dont have replacement n900s :¬( and from the sounds of it thats the only fix.

Anyone know the deal with insurance replacement? Mine states, a similar spec phone of the ORIGINAL value. Thats like £500, i can get a dual core phone at that price.
The spec thing is confusing since theres not many phones as well specd overall as the n900, they have faster processors and more ram but the lack everywhere else. Nothing really compares.

Even camera quality is better on the n900 then most of the other 5 and 8megapixel camera phones. Not only that, the video quality is better on the n900 then on the hd video recording of say the HTCs. for example the htc desire is blurring, laggy and distorted when recording.

Anyway im interested to hear what others insurance replacement experience has been?

I have the same error now after a reboot, it works again..for a few minutes.
could it be that cssu is a problem?

EDIT:
I have recognized.. when i shake my n900, sim communication works again.. for a few minutes

Hmmm my insurance company repaired my phone and are sending it back :¬D now.... whats the betting all they did was flash it, rebooted once it was fine. Just as i did, but within 10 minutes it gave the same error. Im guessing thats whats going to happen again.

Fingers crossed they actually fixed it.
Cant to get my n900 back :¬D

Amazingly they really did fix it!!! cant believe it :¬) No idea what they did. Anyway im chuffed :¬D Would have been nice if they told me what was the problem and how they resolved it so i could tell you guys.

Maybe you could ask them? :-)

Looking forward to read the answer ...

They have no information on what was carried out but they do know they replaced a part. They said they will ask the repair shop to send out a letter with more details, but cant guarantee anything :¬/ Hopefully they will send one out.