I was thinking of getting a Sena Ultraslim for my iP4 but I would also like to be able to use it with a bumper.
Solution: I'm thinking of buying the Sena Ultraslim for Blackberry Bold 9000 or 8800. Based on the dimensions of those phones, the iP4 with bumper (or similar case) should fit fine. I haven't pulled the trigger yet though.
I emailed Sena about the Ultraslim and I was told they won't ship them out for another two weeks in case you were wondering ecause the site doesn't day it's backordered until it's in your cart.
www.sfbags.com has a self-dubbed "Smart Case" made with leather and nylon with a size available that fits the iPhone 4 with a bumper.