I have had the same problem with my x3i on a few occasions. I've had very limited success with the following, at one time or another: soft reset, hard reset, turning it off and then back on, syncing, etc., etc.....
Sometimes one of these things would fix it, sometimes none of them would. Tonight, it happened again. Screen would not respond to taps, even though the buttons and scroll wheel were working fine. I found an old archive on this site where one person said that they read somewhere that lightly squeezing the screen fixed their problem. VIOLA!!! It worked for me, too.
After reading davrol's response above, I think this may have been my problem, at least part of the time. Squeezing the screen probably dislodged the offending piece of dirt. All the previous times, the dirt probably eventually got knocked out due to all the jostling...
Anywho, next time it happens, I'll try running a business card along the edges of the screen...
Thanks, Aximsite community!