There was a horizontal bar that was unresponsive, both times in the same place; On the dial screen it was across the first row of numbers (1,2 & 3). The first time it happened I had dropped my phone and there was minor damage to the bottom right. The 2nd time I had spilt a glass of water onto it killing the speaker, but the dead zone occurred a week later.
My mate has had the same problem as well, without the drop or water so I think it might be a problem with some screens. Just send it in to HTC for repair, the first time cost me $150 SGD (about 75 quid) as the screen was cracked, the 2nd time it was free of charge as there was no physical damage. I'm sure the replacement screen is a lot nicer to, even seems to fit better as there is no flex or movement at the side.