Coming straight from BMW:
The X6 will go on sale in the US in late spring of 2008 as a 2008 model. The X6 will be offered in the US as an X6 xDrive35i equipped with the same 300 hp twin-turbo 3.0 liter inline 6-cylinder engine that you already know from several other BMW models and as an X6 xDrive50i equipped with a new 400hp 4.4 liter twin-turbo V8 engine.
2008 will be a great year. From my experience, this is the first year when so many new models are introduced in the same time in the U.S market: the BMW 1-series, the new BMW M3, M3 sedan, M3 convertible and the BMW X6 and also there is a possibility that a facelifted 3-series will be available in the fall.