I can confirm same kind of of mic problem, it has happened at least once. When the bug was triggered, in all calls the voice of the N900 user was not heard, no matter how many call attempts or different phones on the other end were used. Not sure yet what triggered that bug. Rebooting helped.

A friend of mine says 5800 Express Music, which is a Symbian phone, with its latest firmware update has a similar bug, although the whole phone does not need to be restarted, just attempting the call again is enough to correct the problem. He said the firmware was withdrawn from Nokia Update service and he cannot downgrade.

There has been also a situation with N900 that the phone "dropped" from the network, no call went through. Restarting the phone helped.

I bet 5800 Express Music and N900 are running almost the same GSM/3G protocol stack code (isolated from Symbian and Linux) and the bug is somewhere there.