I boughtan apple bumper case but didn't like it much. It puts a "lip" around the otherwise flat design and the rubber/silicone lining traps dirt. Then I found the Sena ultra slim leather case for $29 at the apple store. It's designed for the 3G but works perfectly for the 4. It's velvet on the inside and so slim, fitting perfectly in my pocket. The top of the iPhone 4 is perfectly flush with the top of the leather case, just like the pictures of the case with the 3G iPhone inside. Check it out online at
http://www.senacases.com/apple/iphone-3g-3gs-cases/ultra-slim-pouch-sleeve/