Huh? If anything, the existence of God threatens the concept of free will. It's kinda hard to feel free if there's some omniscient, omnipotent dude who knows and controls everything that will happen ahead of time...
UPDATE: A brief summary of compatibalism: http://www.rep.routledge.com/article/V014SECT1
According to them dogs do not have free will but people do. Probably talking about ethics and moral side of "free will" question, not physical or religious. But even there "free will" is just an illusion in your mind.
This is a philosophical question, and as such, can't really be answered experimentally. At the risk of sounding sophistical and clintonian, it depends on what you mean by "'really' free". You can be controlled by environmental factors and still exercise free will by some definitions of that term.
Just because God may know what you will do ahead of time, does not mean God caused you to do it.