Winding Road says the Buick hybrid will use a Belt-Alternator Starter hybrid system (BAS) like the one in the Chevy Malibu. This won't result in 50 mpg ratings for the LaCrosse hybrid, but should increase the vehicle's numbers a little. Current LaCrosse models in the US get around 22-24 mpg. We'll have more when GM officially announces the vehicle.
[Source: Winding Road]