By now you've probably updated to the new cleaner looking version of iTunes 9. It looks great, but some people are having a hard time finding some old features. Specifically, I've read story after story about the demise of the Mini Player. Never fear -- it's still there.
How to do it:
1. Open 'iTunes 9'.
2. Go to the 'View -> Switch to Mini Player'.
Select this option again to switch back to normal.
3. If you prefer a keyboard shortcut, press 'Command + Shift + M'. This keystroke will alternate from mini to regular.
4. Or, you can hold down 'Command' 'Option' and click the 'green button'.