I became accustomed to using CMD+Z to switch iTunes into Mini Player and normal view. Not sure why I started using CMD+Z (I'm pretty sure it was the original shortcut before it got changed..?). Anyway, in previous iTunes versions, the green button would take it to and from Mini Player. So, to get the CMD+Z shortcut to work, I created a shortcut in Keyboard (in System Preferences) for iTunes, and assigned the menu title "Zoom" to CMD+Z. Well, that obviously doesn't work in iTunes 9 since the green button, or "Zoom" now just does what the green button does for everything else in OS X. So to get around that, all you have to do is add two new shortcuts. One for Switch to Mini Player and one for Switch from Mini Player and assign them both to CMD+Z (or whatever you prefer). Thought I'd share that if anyone else was in the same boat as me.