OK, this may seem too simplistic, but I've had this problem since day one, and I think I figured out why I was having it from time to time...
I'm left-handed. When I hold the phone to talk, I use my left hand, and my thumb is typically somewhere near the front of the phone. The sensor that disables the display when the phone is up against your head is in the upper left corner of the phone, above the display. When I take the phone away from my head, my thumb is still over the sensor. Duh!
That was definitely the reason for some of my issues. Now, I still think I have the "long call" issue that others are experiencing, and hope they fix it in an early patch.
Overall, this is a terrific phone, the best one I've had in 15 years!