The Griffin Elan leather slipcase is my favorite. So the phone is usually practically naked while in use. Scary, but the iPhone is taking the joke so far. I do have a Power Support Crystal film on the face, you can't even see it's there and it doesn't interfere with operation. Also it has stayed put and the Elan case is pretty snug, so I'd rate that film as a 5/5.
Note that I do NOT use the Elan clipped to a belt -- and I have heard there have been major issues with the clips on that case-- but i did leave the clip on mine because it sits nice and flat; I have turned the clip to use the slot that lets it lie across the width of the case.
Alternatively, the type of black synthetic fabric slipcase that Apple used to enclose with certain iPods also works great with my iPhone. The one I initially used with the iPhone came with a 40Gb 4th gen white iPod, I think. But sometimes I like to use that slipcase with a 60Gb video iPod instead of having that iPod in an iSkin, so I ended up getting the Elan for the iPhone.
I have a black Incase that I occasionally use on the iPhone. It's nice and grippy (thus, not so good for sliding in and out of a pocket) and it's very protective of the iPhone's edges and corners, great for when the phone is in use outside a normally sheltered environment. I have some concerns about its potential for stretching out so that it could end up trapping grit and abrading the iPhone surfaces, but these concerns are are based on other people's posts that I've read and not my own experience so far. I'm careful how I take it on and off and have washed it a few times; it still seems to do a fine job.