Dolby Digital Plus is available on HD-DVD and Blu-ray, and while it is capable of 13.1 channels it is currently only implemented for 7.1 (and, for that matter, there's been no studio support for more than 5.1 so far). It is "backward compatible" from the standpoint that it can be output as multichannel analog or downmixed by the player to Dolby Digital 5.1 (although Toshiba's HD-DVD players are actually downmixing to DTS instead because it requires less compression). My HDMI FAQ (in my sig) has some more details - suffice to say that I don't think we're likely to see Dolby Digital and DTS become obsolete in the near future.