You're right, but let me revise that slightly. In a simulated universe, some forms of self-improvement are possible, but others are cut off. Specifically, all forms of self-improvement which require more resources than you provide in the simulated universe are cut off. The problem is that that includes most of the interesting ones, and it's entirely possible that it will self-modify into something bad but only when you give it more hardware.
At the recent London meet-up someone (I'm afraid I can't remember who) suggested that one might be able to solve the Friendly AI problem by building an AI whose concerns are limited to some small geographical area, and which doesn't give two hoots about what happens outside that area. Cipergoth pointed out that this would probably result in the AI converting the rest of the universe into a factory to make its small area more awesome. In the process, he mentioned that you can make a "fun game" out of figuring out ways in which proposed utility functions for Friendly AIs can go horribly wrong. I propose that we play.
Here's the game: reply to this post with proposed utility functions, stated as formally or, at least, as accurately as you can manage; follow-up comments explain why a super-human intelligence built with that particular utility function would do things that turn out to be hideously undesirable.
There are three reasons I suggest playing this game. In descending order of importance, they are: