According to an article in TV Technology, a consortium of Italian broadcasters and technology companies has come up with a way to broadcast 3D video content over a television channel in such a way that a standard 2D HDTV could still show a high definition image.
The key to the technique is that two 720p images (left and right) can be combined and sent in a single 1080p frame. This can be done in a way that the 2D set would only recognize one of the two images, and display just that content. A 3D compatible set would find the other image, and display both. The advantage of this approach over some other encoding methods is that it does not require that half the image be deleted and then recreated through interpolation. That approach would mean that the 2D image would only have half the image data of a high definition image.
The backward-compatible technique is being reviewed by standards agencies, but it could be a long time before such an approach will be implemented by broadcasters, either in Europe or elsewhere.