Good question. I think BMW will give the 6-series a face lift or some kind of minor update and give it the new 4.8 liter V8. As for the X5, I think it will ride out with the 4.4 liter V8 until they release the redesign, which will probably have the 4.8 liter V8.
BMW is slowly phaising out the 4.4 liter V8 because it is an older motor. The new 4.8 delivers better power, great fuel economy, and superb reliability (something the 4.4 V8 was lacking 10 years ago).