Tidbits has an interesting take on what the SDK will involve. And they seem to have a good source. Of course it isn't a done deal till it's out. But right now the concept they've heard is that it'll be two levels of access. One a more general access that'll let you have apps that can use the screen, touch input, and wi-fi. The more stringent is access to the actual cell phone bits, and that's probably where you'll see much, much tighter control by Apple. Requiring signed apps that are counter-signed by Apple on the cell phone parts seems very likely. So that Apple can allow apps for the cell facilities without worrying about the SIM lock getting cracked by them.

I think the lower security level is where we are going to see a lot more openess. Or at least the potential for it.

P.S. Currently all programs on the iPhone run root. Which is why that TIFF exploit works on 1.1.1. On the upside that means very likely that the phone is going to be jailbroken, and subsequently have its SIM unlocked again with 1.1.2 and so on. On the downside that is also a serious future security concern for all iPhone users.