Not exactly SD either.
For 720p, it`s basically two 640 x 720 images side by side totalling 1280 x 720p. Each eye sees one 640 x 720 image. This way no extra bandwidth is required so it can be broadcast on one channel.
Contrast that with Blu-ray 3D which is really HD because its each eyes sees a 1920 x 1080 images hence why it requires twice the bandwidth.