Just to clarify, it's using the dock connector that's important. This can actually be done without a physical dock with some aftermarket cables. ie:
http://www.amazon.com/Cables-Go-35507-Connector-Cable/dp/B000JG3WBY
http://www.virtualvillage.com/Items/006004-019

That could be useful if you're looking to accomplish a line output for use with a portable headphone amp, in-car use, etc. The dock is a pretty nice accessory at home.

- Jason