A bunch of people claim the coolest thing about the iPhone is that it runs OS X or at least a customized version... however the agreement with Cingular locks the OS to apps of Apple and Cingular's choosing.
$50 says it takes less than 3 weeks after the iPhone goes to GA that someone hacks the OS to run whatever they want.
iPod Linux (Yes, I know it's a different beast)
Update: An indepth look at third party apps and the iPhone.
Update 2: Let the iPhone hacking begin.
Update 3: Early iPhone Hacking Efforts
Update 5: Unlocking the iPhone Software, Hardware, and Lawyers! Yay.
Update 6: Apple finally launches an SDK and iPhone developer program. The catch, all apps must flow through Apple. Apple takes a cut and gets to assure AT&T that none of the apps will create odd network conditions. So long to the IT layer cake.
Update 7: Since technical methods of keeping the iPhone locked to AT&T failed, now you can't buy an iPhone without activating AT&T service. Score: Technology 0, Process 1 for now.