Toshiba and Memory-Tech have just announced a new dual layer disk format with one DVD layer and a second HD layer for high definition video on the same disk. The cool thing is that a disk made this way will play back SD on a regular DVD player, but HD on a player and monitor designed for that format:

http://www.datafuse.net/page.php?news=363
http://www.techtree.com/techtree/jsp/showstory.jsp?storyid=56564