Certain keys won't work

On my 16gb iPhone (2G), all of a sudden...the 4, 5, and 6 keys will not work...it almost appears as that section of the screen isn't responding to touch, as it is happening with all applications in the upper middle of the screen...anyone heard of this before...or more importantly how to fix it? This particular phone was just swapped out for a new one a week ago and has worked flawlessly since...no drops...or water encounters...just all of a sudden...

Any help would be greatly appreciated...

thanks!