I bought a brand new Pontiac G8 GT a couple years back. Not only did I leave the dealer lot without activating OnStar (much to the dismay of the salesman), I removed the module from the car as soon as I got home. Very easy to do, simply unplugged the antenna and electrical harnesses, and unclipped it from the rear deck. No tools needed, no side effects, and the only "feature" I lost was bluetooth integration (which I don't use) and obviously the OnStar "features" which I didn't want. When I traded the car in, I popped the module back in the car. No harm, no foul.
I suspect not all GM vehicles make it this easy - especially from an accessibility standpoint. IMHO it would be welcomed, though. I have heard some vehicles pass the CAN bus THROUGH the module, rather than adding it as another module ON the bus. In this case, you'd have to jumper the harness, but still easy enough for any /.'er to accomplish I imagine.