Problem with returning to iTunes when in full-screen mode
I've had similar problems with other applications running in full screen mode. This is not a problem with your install per say, it's a bug/feature/the way things are. Don't do anything drastic like reinstall the OS because you won't solve the problem.

When this happens I generally move back to the full screen app using Quicksilver. You can do the same thing using the built-in spotlight search ⌘ + spacebar or through Mission Control.