Not really a new idea, and it is pretty obvious in its implementation (I had the same idea about games for the Mac a while back). I'd love to see it happen. But the problem, of course, is just how open Steve wants to (or can be) with the iPhone SDK. I really doubt we're going to see something freely distributed, and I know for sure that we probably won't see anything that anyone can place unsigned content on. I'd love to see a quick, easy way for anyone to simply publish apps for the iPhone, but from what I've heard, there are so many security holes in the system right now that that's not really a possibility.
Jobs clearly wants to fix things (if his iPhone SDK note is to be taken honestly), but unless Apple starts plugging holes fast, I don't see it happening. More likely, I'd see a few trusted developers given SDKs, and asked to create applications of their own which might then be listed on iTunes, for sale or download. Other developers could probably apply, but Apple will likely try to keep control over the whole thing, pulling an application off of iTunes (and possibly even the iPhones themselves) when a problem is discovered.
It'll be interesting to see how this all plays out. As I said, it would be great to see apps on the iPhone from anywhere and everywhere, but the concern we're hearing is that security is far from tight, and Apple is watching its back as closely as it can.