IF the new dock changes to micro-USB, the 3.5mm socket could contribute the direct audio/video ouput that USB can't. When docking, the bottom of the iPhone would mate with both a USB plug and 3.5mm plug (which would also keep it upright). Older iPods used to output composite video through the headphone socket, so it might seem like a strange step backwards, but also possible and practical. I'm speculating, but not entirely irrationally.
There is actually a really good point here. On a lot of the "less expensive" sound docks, the 30 pin connector does not do a very good job of holding the phone in place to keep it working properly. The 3.5 mm jack would be a nice post to keep the phone secure and consistently working.
My wife has an Altec Lansing sound dock that she has to rig with a pen behind the phone to keep it in the precarious spot required to play the phone properly.
I had initially though that this little dock connector (or whatever it will be called) would be put under some serious stress when a phone is resting on it alone.