It would have been playable in both HD-DVD drives and Blu-Ray drives!
Scared by the prospect of being the only producer of the format, they claimed it could "be hard to make it go."
Warner will continue to release their movies on both formats... for now.
for the whole story
http://www.engadget.com/2007/09/15/warners-blu-ray-hd-dvd-hybrid-disc-on-indefinite-hold/
