If I change it to the bus ID of the 9400M integrated (4:0:0), I then get an nvidia splash, the x server starts, gdm, gnome, etc. This is much better than a console, but it'd be really great to be able to use the dual 9800Ms.
This leads me to my next question: Is this setup actually correct? Will the driver somehow switch to the 9800M's under stress? All of them show up in the nvidia settings control panel in gnome.