If you try to delete a playlist in iTunes, a confirmation dialog pops up and asks you 'Are you sure you want to delete this playlist.' To get rid of this warning you can select the 'Do not ask me again' check box in the dialog, and iTunes will happily allow you to delete playlists without asking you first.
However, if you want the warning in general, but want to bypass the dialog on a one-time basis, just use a Command-Delete keystroke. iTunes will then bypass the warning for just that playlist and delete it immediately.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030530215933594