Ah.. a bug.

The gap is necessary because the active area of the touchscreen extends under the case; if it actually touched, pressure on the front panel (from gripping the device) would probably register clicks on it's own, or even if it didn't, would "bend" the position of other clicks...

But there's really quite a lot of room under there; thoughtfix has some disassembly pics and video in his blog, and it's really quite easy to pop the front panel off for routine cleaning. All the waterproofness is inside that, so the bug can't get anywhere harmful.