The latest release of iTunes (10.1) adds some extra features -- adding and removing Ping and Genius is back.
You can now selectively remove Ping from the Sidebar using the General tab of the iTunes preferences. And for those of you (like me) who mourned the loss of Genius from the iTunes Sidebar, it's back.
[crarko adds: I tested this, and it works as described. Nice to see that Apple is listening to some user feedback.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20101112201922800