I've had this issue too. I accidentally hit face time, speaker, and even hung up on someone with my face during a call.
I noticed the same issue too but someone on here posted to try doing a "reset all settings" on the phone. Did that, and haven't experienced the issue since. Give it a try....
I did that- as have many on the Apple support forums. But it didn't make a difference. I still am face dialing.
For now, one workaround is to hit the sleep/power button as soon as you start a call. Then the screen is turned off.
just talked to apple tech, she seems to think its a software issue and had me restored the phone as new to try it out for the next couple of days. She said she hasn't heard anything about this issue from customers or her colleagues
Don't you love when Apple Store people or Apple tech support says that they never heard of a problem- but the problem has been discussed on both forums like this one and Apple's own discussion boards at length!
When I picked up my iPhone 4 on opening day, the Apple store person told me they never heard of the face sensor issue, or of the yellow screen issue.