I have about 1,000 songs in my iTunes library and about 200 of them are unchecked for one reason or another, and I wanted to make a playlist that included my checked songs only. To do this, I simply went to the File menu, and selected New Smart Playlist. In the window that pops up, the first condition I chose was "Date Added / is after / 01/01/95." That condition includes all the songs in my library since they are all added after 1995, and then I simply checked "Match only checked songs." I now have a "Checked Songs" playlist that updates on the fly, depending on if a song is checked or not. Works out nicely.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040326022532793