Another new AppleScript track property in iTunes 7.1 is unplayed. This item does not appear in the GUI (not even in Smart Playlist criteria!?) but is accessible in the XML file, so I suspect this is something AppleTV may want to know about. In any case, you can set a selection of tracks as unplayed or played by setting the unplayed property to true or false, respectively. And that's what Selected Tracks Played or Unplayed does for a selection of tracks. Need I add for iTunes 7.1 or better only?
UPDATE: it appears the unplayed property may be specific to podcasts. However, the script will work with any track.
MORE UPDATE: do my TV Shows now have a blue dot on the far left indicating their played status?