This applet will export each selected plain, Smart, and Genius playlist as individual XML or M3U files to a user selected directory. Such files can be imported into iTunes again later. Not as fast as iTunes' own "Export Playlist..." command, but it does the job.
For OS X 10.6 or later.
Also see Export Selected Tracks to XML.
Select some playlists, choose the file type, and click on "Export".