In this presentation from the recent Toorcon 15 in San Diego, CA, Michael Ossmann and Kyle Osborn detail their efforts in hacking cellphones via multiplexed USB signals.
Manufacturers of mobile devices often multiplex several wired interfaces onto a single connector. Some of these interfaces, probably intended for test and development, are still enabled when the devices ship. We’ll show you how you can get a shell on a popular mobile phone via its USB port without using a USB connection and we release an open source tool for exploring multiplexed wired interfaces.
Link to the tools referenced can be found at GreatScottGadgets.
I added some more things to the page linked above and also made a short blog post about this stuff.