This is interesting -- Alex Winston, Ltd. has come up with what they call the Wolfenstick, a little iPhone app that can control Wolfenstein 3D Classic with a second iPhone through the audio port. Obviously this isn't a very practical setup, as buying a second iPhone just to control games on the first one probably isn't in anyone's interest. But it does open up a whole slew of possibilities for how a connection like this might be used -- as you can see in the video above, not only are controls and the accelerometer able to be transferred, but even game and app information can be sent across the cable.
Of course, the 3.0 update casts a nice large shadow across ideas like this -- we know for sure that there will be more hooks for accessories to work with apps on the iPhone, so having to go through the audio port may end up being more of a novelty, as it'll likely be much easier to go through the connector on the bottom of the iPhone. But if you want to play while the unit is charging (or just want to connect two iPhones with nothing more than an audio cable), there's definitely some promise here.