GM seems to provide an oil plug torque value in their manuals, at least in the couple that I checked. They even gave a torque value for the oil filter, which I had never seen before.
I just did an oil change on a 2004 Chevy pickup with the 5.3L V8 that has a cast aluminum pan. I tightened the drain plug using the "snug" method. After this thread started, I looked up the torque spec in the manual (18 lb-ft). For the heck of it, today I checked the torque on the plug I had "snugged" by progressively increasing the setting on my torque wrench until I reached the setting where the plug finally budged. It turned out to be around 36 lb-ft or so--double the spec. I guess I have to ease up on my snugging. Even at 36 lb-ft, it certainly wasn't as tight as it was when I removed it.
OK4450 brought up the issue of possible liability if a shop were to repair stripped drain plug threads using a Heli-Coil. I just discovered that many of the GM LS-series engines with aluminum pan came from the factory with some sort of thread insert (presumably some type of Heli-Coil) for the drain plug.