I have not tested if the cable will fit with the bumper, but if it does not I will just keep one of the white cables with me since they fit and plug it into the usb.
Sand the connector down with a dremel or use scissors to shave the connector until it fits, i had to do this with my spare usb cord i use to charge my iphone at work......
I have the Apple Composite cables hooked up to a Pioneer in dash DVD player and I get audio/video output but it's real touchy about charging. If I barely touch it or move it stops charging but doesn't affect playback... I had to use a Dremel to sand down the connector so it would fit inside my old Otterbox Defender equipped 3G and 3GS. Worked perfectly cause the fit was nice and snug. Hopefully the new iPhone 4 Otterbox Defenders have the exact same size connector port hole.