I had the same issue. I was listening to a podcast last night with the iphone earbuds. Later i did the software update, which required a full restore because i had modded to add ringtones. This morning i went to place a call and couldn't hear anything out of the ear speaker on the face of the phone. The phone said it was on a call but i hadn't heard dialing or anything else. I put the call on speaker and found that the other party was there but hadn't heard me and I of course hadn't heard them. This actually happened on two different calls. I thought i was going to need service for sure.
I did try a reset of the iphone but this didn't help.
I found this on the apple support discussion site.
http://discussions.apple.com/thread.jspa?messageID=4985099�
Basically that thread is other folks who had the same issue and they resolved it by plugging and unplugging the headphones. I tried this with standard (no mike) ipod earbuds and had to do it several times but it did eventually work. And now everything is working fine.
I'm guessing this must be a hardware issue and not software because I hadn't used the earbuds at all after the forced reload during the update. A friend also said that she had heard this happening with the tmobile sidekick as well.
If it happens again, i'll definetly be talking to my local genius. ;-)