Combo and Twin discs already exist
HD-DVD has had combo discs from day one. These are discs with HD-DVD on one side, and normal DVD on the other. This means you can start building your HD-DVD catalogue now, even if you have a standard DVD player. This is the format the new Star Trek: Original Series Remastered (with new CGI) is being released.
HD-DVD also has "twin" discs. This is a dual layer HD-DVD with HD-DVD on one layer and normal DVD on another layer. This means that unlike the combo disc, you don't have to flip it over to change between HD-DVD and DVD. This gives you a 15GB HD layer and a 4.7GB DVD layer.
Bluray has some serious yield problems at the moment - in order to increase yields, they have to reduce the capacity a wee bit (this is due to the spin coating - it can become uneven around the outer edges, and because of the high tolerences required for Bluray, it can make it harder for the laser to focus here. This is becuase Sony decided on a higher numerical aperture for the focusing lens on their blue laser pickup in order to cram in more capacity, which means the data layer has to be just 0.1mm from the surface of the disc, which needs new technology to protect it).