I completley disagree with those who say it will help or speed up the cycle. I think there is a bit of misunderstanding by some people posting here as to how the cycle happens in a new tank. I'm not going to type the huge post necessary to explain this, but suffice to say that there is a TREMENDOUS amount of bacteria already living on the new live rock - that's why you use it. A bit of water from an established system is going to do next to nothing when compared to what the live rock lends to the system. The bacteria live on surfaces, and realatively little of it exists in the water column. Furthermore, if there is detectible amounts of ammonia in the water from the established tank to aid in bacteria growth, something is VERY wrong with your established system!
Live rock as an incredible array of bacteria and living organisms on it, THAT is what helps your tank cycle. Forget about adding water from another system. If anything, add some sand from the existing system.
Cheers
Jim