Got the goat in the shop today, and its pretty much like I thought. The tech tells me that the "slip ring" in the ignition switch is intermittently not communicating with the key. The exact condition is described in the service information he got off the GM service computer, so we're going to replace the ignition switch, ignition cylinder, 2 keys, and 2 remotes to make sure its fixed. I had to wait about 3 weeks for the cylinder to come in, and I'm the Parts Manager here, and know the shortcuts to get backordered parts, so the cyl's are definitely on a hard backorder. I must have gotten 50 calls from other dealerships wanting to buy that lock cylinder!
Anyway, once its fixed I'll give an update.