I'm still debating what to do myself. My 3GS doesn't have much resale value currently due to some glass damage, so for me it's not too much of a concern if I go back. I would however miss the iPhone 4 features, as it's really grown on me already. I have a trip coming up, and I think I'll make a final decision on returning or keeping it after that.

I am going to return the bumper case though. I can't plug the iPhone 4 into my temporary speakers in the car with the bumper on, nor can I plug it in via the dock connector. Plus it's nearly caused me to drop the phone already, due to the rubber gripping the inside of my pants pocket as I'm removing it.