I was looking at the Chevy site last night for this very reason. The avalanche has this option and the demo video showed the 2,3,5 & 8 cyl's firing for ballance. I assumed it would alternate from the previous to 1,4,6 & 7 but the demo didn't show that. I also noticed that in the standard package of the vehicle you get a flexxible coupling from the engine to the exhaust for quieter opperation... Makes me wonder about how 'seamless' the 4 to 8 change is. I also didn't see much on actuall fuel savings.
I'm interested in this, could be an option for a good size truck that gets decent milage but I want to find out a lot more so that in 2-3 years I'll know if I want one. (or whenever the price get's closer to 20k w/ a few miles on them)