[FredZarguna]

If the General Theory of Relativity is correct over large distances [and there is no experimental evidence to believe it isn't] the speed of gravitons cannot exceed c. There are a lot of references in the literature. Among many, here is one accessible to laymen: http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html

[SubMareener]

Right. So if they are detected going faster than C then General Relativity would only be an approximation at solar system scales.