Given the relative simplicity of the iPhone platform, compared to Mac OS X, Apple shouldn't have that difficult of a time rigorously sreening potential third party apps for the iPhone. I think Apple wants to be the main if not sole distributor of iPhone apps, not because they want to corner the market, but because they want to control the quality of everything that could potentially reflect on the perceived quality of the iPhone.
And how much do you think it'll cost for Apple to QA every 3rd party app?
Whos going to pay - Apple or the developer? If developer, say good bye to a large amount freeware / shareware apps.
Apple should quit being so controlling. iPhone will only benefit by having an unrestricted 3rd party environment - just like every other phone platform - smartphone or not, has.
Apple's controlling nature killed the Mac platform from being a main player in the the computing world ( Mac still has a very small minority of users at 6% ), and it'll kill iPhone too in the same way. iPod is different - you play music and that source is readily available e.g., rip CDs.