In previous versions of iTunes, to check/uncheck multple selected songs, I would have to use the Enable|Disable Selected Tracks script (visible on this page) from Doug's AppleScripts for iTunes.
I just noticed today that if you right-click (or control-click) one or more tracks, within the context-sensitive menu is an option to Check Selection if the selected tracks are all disabled, Uncheck Selection if the selected tracks are all enabled, or both commands are displayed if the selection is not all checked (or all unchecked). Excellent!
[robg adds: I hadn't noticed this new choice on the contextual menu, and given how its behavior changes based on the status of the selection, I thought it was worth pointing out...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051001070207944