Same here. If you put you finger over the proximity sensor and then remove it, so that the screen turns on and of, you should be able to hang up. This is annoying though. (proximity sensor is in the top left corner)
My guess is that it has something to do with the sensor and the screen turning off and a timer. I have this problem at times, and my Palm Pre also has the same issue.
I've gradually just learned to look at the screen and wait 1 second after it turns on before hanging up.