Toshiba has been tinkering with the HD DVD and DVD disc formats, putting together a hybrid format with three layers that contain both formats. The DVD section will be playable on conventional DVD players, and with a firmware update, standard HD DVD players can play both formats.
So it looks like we'll be seeing either a single-layer 4.7GB DVD along with a dual layer 30GB HD DVD, or conversely, a dual-layer 8.5GB DVD and a single-layer 15GB HD DVD, all together on one disc. One more layer, lots more content.
This idea could add more capacity to those already-existing single-layer hybrid discs, where a movie is offered on DVD for use today and also includes a high-def HD DVD copy on the same disc for those who think someday they may be suckered into buying an HD DVD player. However, this is at cross purposes with movie studios, who prefer the idea of selling the same content over and over.