There's a box you tick (or untick) when you do the initial setup (right after the factory reset) that says "stay awake during charge" or something to that effect. Maybe you neglected to untick that box???? Just wondering...
I'm having the same problem with my new HTC one x. What's worse, I can't even hear the caller using a headset or putting it on speaker. Thinking I'm going to have to return my phone. :(