In previous versions of iTunes, clicking the Zoom button (the green +) on a Mac launched iTunes’ Mini Player.
iTunes 9 changes the behavior of the Mini Player on a Mac.
There are two ways to access the Mini Player on a Mac:
• Hold down the Option key and click the Zoom button
• From the View menu, choose “Switch to Mini Player” (⇧⌘M)
Source: Apple support article HT3871.
MacDailyNews Note: To keep the Mini Player in front so that you can always see it no matter which application is active, choose iTunes > Preferences, click “Advanced,” and then select “Keep Mini Player on top of all other windows.”
[Thanks to MacDailyNews Readers too numerous to mention individually for the heads up.]