Unfortunately, no iPhone for me (yet-Verizon).
First, conceptually no reason why you cannot take audio and lay it onto an iMovie segment, or take a video shot on an iPhone and re-edit with iMovie. But, in the Wonderful World of Video things are not always as they seem. So you may need the help of tools and utilities to modify/transform/encode things the way you want them. These include handbrake, QTime7 Pro ($20), Perian, VLC, Flip4Mac. QT7 Pro (not the QT X that comes standard with a new iMac) performs lots of useful Save As and Exports that can really save the day. I don't think you'll need QTPro for this, but when you do, spend the $20 -- well worth it.