I just went through the case program for my brand new iPhone 4, but I'm still poking around to see if there's one worth buying that I might also pick up in addition to the free one. Like, for example, this awesome audio cassette case from a company called Fred and Friends. It looks like an old plastic audiocassette, but it's actually made of silicone, and it even comes with a little tape box (oh man, I had so many of those back in the day) that flips around into a video stand.
Very, very cool. It's not available yet, but the case is supposed to ship in October for US$20. Yes, if you have an iPhone 4, you've already got a free case from Apple, but man, something this cool might just be worth the extra money anyway.