I've been on the lookout for a case since I got my iPhone on 7/14. I found two at very attractive prices at Walmart of all places. The first is a hip case style with a belt clip made by BodyGlove:

It fits the 3G very snugly and adds less bulk than other hip cases I've looked at. The best part is that it was only $15.

The other case is a neoprene pouch with a carabiner clip made by American Tourister. That case was only $9. It is a bit short for the iPhone but it will be handy when I want to hook the phone on my computer case or camera case. I haven't been able to find any info on the web about this case. The package calls it "Cell Cushies" but google searches find nothing.