Uh, sorry, no. You can watch a Blu-ray disc on a standard definition TV right now. No problem. Blu-ray players have a legacy standard definition output on them for backwards compatibility sake, it will downscale the 1080p to 480i no problem, scaling from a higher quality source down is a trivial matter. Sure you won't get much picture quality benefit from it, but it works fine.
I didn't say you couldn't. I said you won't get the extra definition from it, and you'll probably even get a worse definition than what you'd get from DVD, in the case of BluRay.