Ok, here was my deal. It may be different for others, but I got this solution with the help of some nice Microsoft GFWL engineers.
My Killer NIC was preventing me from reaching the LIVE service. I was skeptical of this at first, but they asked me to uninstall the Killer NIC and then try the onboard Ethernet. Well, after uninstalling the Killer NIC, I couldn't get either onboard Ethernet port to work. Oh, they said they worked, and that I had an internet connection, but I really didn't. I tried for a couple of hours to get those onboard ports to work, then I threw in the towel and reinstalled Vista.
Long story short, fresh Vista install, using NVidia Ethernet drivers and motherboard NIC, Game For Windows LIVE now working fine. I just enjoyed an hour of Shadowrun with some Xbox 360 players, got some nice achievements, kicked some butt.
I notified the people at Bigfoot networks already, hopefully they can come up with a driver solution, or Microsoft will patch the game, but I'm in for now, so the Killer NIC is getting shelved until the issue is resolved.
I'm guessing 0x8015193 is a NIC error code, so even if you don't have a PCI NIC, you could maybe try reinstalling your Ethernet drivers, or getting a cheap PCI NIC card.