<p>Any pointers on how to initiate serial communication with the iphone?  Or any other idea to interact with external hardware?</p>

It depends on what you want to do. For an SSH terminal connection I reccomend TouchTerm (search the appstore).
I have no experience with electrical connections, but you can find the pinout of the iPod/iPhone connector here:
http://pinouts.ru/PortableDevices/ipod_pinout.shtml
You can then download the iPhone developer kit here:
http://developer.apple.com/iphone/