While it's not obvious, you can use the MiniPlayer with full screen mode in iTunes 11.
Switch out of full screen mode, set iTunes to show on all desktops via the Dock, open the MiniPlayer as a separate window (Window > MiniPlayer), then click the full screen button on the main window.
I currently have the MiniPlayer sitting in the upper-right corner of my 17" MBP's screen so that I can quickly access it while working on my 27" LED Cinema Display with iTunes running fullscreen in the background.
[kirkmc adds: This works as described. However, if you have the MiniPlayer set to float above other windows (in the iTunes Advanced preferences), then it won't display when you switch to iTunes. This makes sense, of course, because you don't need it when there.
Also, this is the only way to get the MiniPlayer to work across spaces, if you use them. If iTunes is not in full-screen mode, then the MiniPlayer only displays in iTunes' space.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20121129214020214