Unwiredview.com found an Apple patent application for a "dual-sided trackpad device," which resembles the current iPhone redesigned into the clamshell format so popular with many mobile phones. The key to this design is having touch-screen capabilities on both the top and bottom of the phone when it's open.
This design goes a step further, as well, in that the closed cover of the iPhone could also have trackpad capability. PC makers have experimented with this, adding some basic buttons and capabilities to the covers of closed notebooks. Apple's patent application describes how you could use the closed iPhone as an iPod, with controls for advancing to the next track, adjusting the volume, and so forth.
It's quite possible that Apple could be considering other form factors for future generations of iPhones, but I wonder how the touch-screen keyboard and the video player would work on a clamshell model where the one big screen is now divided across two panels. Patent applications aren't necessarily indicative of product plans, but they often provide hints of what companies are thinking about down the road.