The application that is downloading and executing code would itself have to be signed, so this doesn't get around the App Store approval process: the bootstrap app could be loaded by anyone with a developer certificate, but then they can just sign anything they want that runs as a normal process.
The effects this would have are in:
a) security (although codesign is a fundamentally flawed system due to return-oriented programming techniques and is frankly just security theatre at this point)
b) meta-store fronts (Apple would have to be much more careful about approving applications that downloaded other applications at runtime; Apple could revoke these, though, and you can already do stuff like this with interpreters, although you might not want to bother)
and, I'd say most importantly, c) JITs (suddenly there would be no technical obstacles to having good versions of Java, Mono, or even Flash; and given that the contractual obligations they added got so much pushback that they were removed, we /would/ see good versions of Java, Mono, and even Flash)
The effects this would have are in:
a) security (although codesign is a fundamentally flawed system due to return-oriented programming techniques and is frankly just security theatre at this point)
b) meta-store fronts (Apple would have to be much more careful about approving applications that downloaded other applications at runtime; Apple could revoke these, though, and you can already do stuff like this with interpreters, although you might not want to bother)
and, I'd say most importantly, c) JITs (suddenly there would be no technical obstacles to having good versions of Java, Mono, or even Flash; and given that the contractual obligations they added got so much pushback that they were removed, we /would/ see good versions of Java, Mono, and even Flash)