No. Blu-ray is in no way backwards compatible with old DVD players. HD-DVD....... actually I am not sure, but I think it is backwards compatible. Either that or I'm just confusing it with SACD's (which are in fact backwards compatible with old CD players).
Actually, I just looked it up. Some HD-DVD titles have both formats, but not all:
http://en.wikipedia.org/wiki/HD-DVD#Compatibility