From what I understand, that is an assumption, not a fact. Its assumed because nothing is supposed to travel faster than light, but others assume gravity propogates instantaneously.
I will have to look up the specifics.. but an expirement was done some time ago, which had something to do with Jupiter occulting some star.. in which the observations indicated that gravity propgates at the speed of light.
It's been so long (years) since I read that article, I will have to try and search for the info again..
Again I could be wrong, because it's been a while... but that is how I remember it. Will post if I find it, you might be right.
Bones