Interesting - I have an iPhone 4S, and I've noticed that if it's in my pocket, the touchscreen is really unresponsive. I get a call, pull my phone out, and it takes several tries for the "swipe to answer" gesture to succeed (usually the iPhone will drop my swipe about halfway through).
The incoming call probably makes this worse, when I am not in a call I don't recall ever having trouble unlocking my phone with the same gesture.
Was the screen facing your body when it was in your pocket? You just had a very large capacitive body next to the screen for an extended period. What you described would make sense if the iPhone's screen recalibrates itself.
The incoming call probably makes this worse, when I am not in a call I don't recall ever having trouble unlocking my phone with the same gesture.