That sounds like a proximity sensor/transistor issue. Try installing the file aaf709 suggested and see if that fixes things. If not, try cleaning your screen (with the phone powered off) with a light mist of glass cleaner. If that still doesn't work, you could either take it back and demand a replacement or try rooting and seeing if another ROM or a rollback to 2.1 would fix it. If you try the last, don't despair, just backup all your data, wipe the phone clean, and then take it in.