JVC has developed discs that can contain data stored in both the DVD and high-capacity Blu-ray Disc. Currently read-only and still in the developmental stage, the disks are able to contain data in both formats, and actually contain not two but three layers. The upper layer is a Blu-ray Disc layer with a capacity of 25GB. Below are two DVD layers with a combined capacity of 8.5GB.
Similarly, Memory-Tech has developed the same thing almost, except that it marries the DVD and HD-DVD (High Definition/High Density-DVD) formats. These disks contain two layers, an upper DVD layer with a capacity of 4.7GB and a lower HD-DVD layer with a 15GB capacity.
As to what format will eventually win, and find its way into players and computer data drives is currently anyone's guess, but we should know more when these formats and devices for them hit the stores at prices we can all afford.