I was thinking about this the other day, and I think it may hold some wieght. Supposedly gravity has infitite speed: its field is felt everywhere regardless of the length of time it's been around. So, if we could manipulate gravity to form an artificial gravitational field, you could theoretically use a Morse code like system of gravitational oscillations to transmit information to a reciever on another planet. Or, you could just shake an object back and forth and the fluctuations of position could be measured and interpreted. Of course, the technology needed to pull this off would be very complex and sensitive, but theoretically could it work? I think it's more feasable than a rod light years long. :)