Demand, Difficulty... Artificial Limitations?
So far, the barriers of demand and difficulty don't look like a problem for the iPhone. What about artificial limitations created by Apple?

Apple has quite pointedly stated that it believes that an unmanaged amalgam of shareware and open source would result in a phone like the Palm Treo or WinCE / Windows Mobile phones, where security is problematic and system instabilities are a common problem.

That's simply difficult to argue against. While the idea of a Linux based PDA phone that runs everything and burps out source code on demand sounds great and is politically correct, there's a reason why such a thing is conspicuously missing from the market: it makes even less sense than Linux on the desktop.