Resolution isn't actually a huge issue. The goal isn't to alter the field in a single pseudo-capacitor in the grid, but rather several of them. The iPhone's touchscreen controller has to deal with EMI and false positive touches, so it basically scans for "blobs" or small regions of change on the grid. If the blob is large enough and the changes large enough, a touch is registered in the rough center of the blob. All this is done in hardware, and is probably a close-guarded secret, so the specifics are hard to come by.