It's nothing like becoming an Xbox or Wii developer. I've been through all the processes and becoming an iPhone developer is cake. It's not completely open but it's easy enough. Mark could very easily develop a version of Blitz for the iPhone just as a bunch of Garage Gamers (not even official employees) and the Unity crew among others ported their work for it.
Porting and developing for it isn't an issue. Becoming a certified developer that can actually publish a binary for the phone is the issue. There are a lot of rules you have to follow. Certain types of middle-ware are not allowed. There's a size limitation at retail. A whole lot of things very similar to the limitations of developing for the consoles.
So.. it's almost EXACTLY like becoming an XBox developer. Both offer the SDK's for free, both decide if your game will appear in their retail portal.
One might argue that XBox is easier in some ways. At least if you're willing to pony up the dough, other Creator's club members can see your game (even if it's for free).
If your app is not approved by Apple, the only place others can see it run is through the emulator.
I've heard quite a few developers have already been burnt by this. Apple has given the impression that anyone can write to for their mobile platform and make money from it. (Sort of the same issue the Pickford's have recently had with Nintendo)
They got rejected after spending a ton of time, money and resources developing for a platform that has a fairly non-portable API. Even XNA/C# would allow you publish on Windows/Zune if you were rejected for the XBox Live Marketplace.