This tech isn't far off. In fact, it's available now. Infinity Storage Media has released the first hybrid disc, which allows just that. By using a clever dual-layer process, DVD drives will be able to read a DVD layer, while Blu-ray drives will be able to read a Blu-ray layer. The image above shows the process in a rather simple way.
We're not sure how far this technology will reach, or if it will be adopted by other companies. For now, this is a strictly Japan-only product. We can see tons of studios capitalizing on the HD transition by releasing their movies on a hybrid disc like this.