Had the same issues, including what everyone was talking about in the other thread: screen turns off after call connects, as well as whenever voicemail plays back (though the issue never pops up when the speaker is on, but that is normal since the proximity sensor is bypassed when the speaker is on). In the thread that Sasha linked to there was a post by Kinolay talking about wiping the cache partition to clear the problem. I was getting so frustrated because if I navigate away from the phone app when on a call and let the screen timeout, I can never get it back on until the call ends. But I can't hang up! The other party has to, and heaven forbid if it's any sort of automated system or someone's voicemail because it takes F O R E V E R for the other party to hang up.
I've done a few hard resets (I always do one before I update and after. But not the lame way - I go full ##786 and force it to re-activate as well. I love a clean install!) and finally wiped the cache the other day. Still had the same issue, though not as bad in brighter light. Took it to Sprint, waited for an hour and I'll be dammed if the guy didn't tell me it was my screen protector. I nearly punched him in the throat! But then I realized he was right. I had the Zagg on there, but only because it was a freebie when I got the phone. I actually hate Zagg. Lo and behold, there was a recall on the original screen protectors because the cutout for the prox sensor was too small. You can see the sensor yourself if you look at it, but shine a flashlight there and you'll actually see the second one right next to it. So I got a new Zagg for free and the cutout is twice as big. No issues whatsoever now with the phone app or the voicemail app.
I know there are still lots of folks that are having the problem that have a naked phone, so I can only surmise that the sensitivity of the prox sensor has been turned up too high, but maybe wiping the cache partition is the solution until HTC pushes a maintenance release.