Me and a friend are in a debate over the existence of a car called the BMW X6. We both acknowledge that it is in development for release in 2007-2008, but I say that is the first time BMW has used that moniker on a production vehicle. She thinks it was/is the name of a low production volume SUV/crossover vehicle, not sold in the US, or only sold to limited markets. So what is the history (if any) behind the "X6"?

yes. the bmw x6 had been announced in 2006. it'll have a 5.9cc engine and its larger than the x5. very slim shape and attractive.