Upgrading to Snow Leopard made me disappointed to see that Sound Source no longer works as a menu extra. However, this left me disappointed for only a few seconds, as I figured out that by Option-clicking the OS X Sound icon in the menu bar, you can choose your inputs! I don't recall Leopard ever doing this, or seeing it in the new features of 10.6. Enjoy!
[robg adds: This is definitely new in 10.6, and a welcome addition.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090829091221581