The Mobile In won't attach securely to my iPhone 4S because of the case I'm using. I would like to use a Dock extension cable, but I'm confused about which one to get. The specs of the various cables on sale typically aren't clear, but it appears that many cables don't support all of the Dock's functions.

Can you state exactly what the requirements are for a dock extension cable to work with the Mobile In? Can you provide sources for cables that are known to work? I need one with a slender connector body so it will clear my case when I plug it into the iPhone.