I would agree that any aftermarket ECM tune could, and most likely would, void the GM warranty, even if you had your dealership do the install. I consulted at length with my dealership about the tune (Westers I believe) sold by Saturn Motorsports, and my dealership is the local source for Saturn Motorsports. LMAO. But when official word came down about the warranty issue, I got the "Well technicality it could void the power train warranty, but we have never had any issues (from SM, and GM agreed)", my local dealership was not willing to cover.
But with the official GM produced and installed tune, all indications are that the GM warranty is not effected. If memory serves, they did the same thing with an upgrade for the original Ion Redlines. Perhaps one of the GM guys could chime in here with some official GM publication stating warranty preservation? Or refer us to someone that could provide us that documentation. It did sound like there may be some kind of literature in the kit packet addressing this issue as well.