For those who don't want to add the bulk of a case, but want the extra scratch protection, here's my post from another thread:
In case you missed the gizmodo post on the subject:
Gelaskins is already selling skins (with pre-cut holes, mind you) that also supposedly alleviate the signal issue by covering the outer edge/antenna. If you know gelaskins, you know they already have some pretty nice designs from various artists printed, also allowing you to submit your own custom design. Once I get my iP4, I'll strongly consider this as I hate the added bulk of cases, bumpers...
Link:
http://www.gelaskins.com/store/skins/phones/iPhone_4