I can't speak to the specific differences, but I do know that the suspension setup is different for the 19's. I was trying to get my hands on an Acadia that had the color combo and options that I wanted, in time for a family trip, back in early May, and the dealer located one that had everything I wanted, but had the 18's. After much wrangling, I was told it was not adviseable to switch out the rims because the suspension was different. At that point, I custom ordered an '07, right at the cutoff of May 9th, even though I warned the (clueless) salesperson that the order would likely not go through. Can't complain, though, I ended up with the same deal on an '08.
As far as I know there is no difference in the suspension set-up from 18" to 19". The talk on that Saturn thread is hearsay at best.
Now- if you upgarade to the GM accessory 20" wheel (RVO25), then you are supposed to also buy and intstall a "stablizier bar" for about $140 or so. Since this part number is independent if you are coming from 18's or 19's, I can't imagine there is difference in the original factory set-ups.
It's actually all the same, as far as number and type of pieces, I believe it's JUST a THICKER rear sway bar. I.E., the models with 18's have one too, it's just made a hair beefier for the larger wheel models.