tso, I think that blue-ray disks do provide a lower resolution stream than the HD one (not sure, I don't own any blue-ray media or devices myself).

The idea of having different versions of the media to make it backwards-compatible is really logical, since the relative storage requirements for lower resolution content are an order of magnitude smaller than those for the hi-res one. However, this idea has not penetrated much the tech world; for example, Skype (who are now introducing HD video telephony) are saying that it is going to be two-way only, i.e. unless I have 3Mb/s upload speed and a hi-res videocam, I cannot see the other end in HD (even though my download speed is sufficient and THEY may have the proper video equipment). Similar logic applies to the long-delayed video telephony in the mobile devices: while they function perfectly well within the 3G video calls, Skype says the resolution is inadequate to use together with a desktop client, which is total BS because it could be asymmetric just as well... I guess other companies aren't much smarter than Skype in this regard...