Edit: I think a better hypothesis is that Apple wants to be able to analyze app code, and thus disallows executable memory, which also rules out JITs. See: http://apple.slashdot.org/comments.pl?sid=2044470&cid=35...
The propblem is that if you allow executable memory you open aup a lot of exploit vectors. Now, that's true of Mobile Safari as well, but they trust their own team more than they trust you, and they have one app to watch instead of 100,000, and if an exploit os found they can push out a fix immediately, whereas 3rd party apps have a tortuous process for pushing out bug fixes.
Remember, it's part of their value proposition that apps from the app store appear to be relatively safe in comparison to the clusterfuck that is downloading random desktop apps, especially on Windows.
So you could have a buffer overrun anywhere in the app. For example, if they are silly, you could go to the preferences for Facebook and enter a very, very, very long user name, overrun the name buffer, and have executable code.
That's not an exploit of UIWebkitView, it's an exploit of giving the application the permission it needs to have UIWebkitView use a JIT compiler for JS.
