Well, I played quite a bit of Portal last night. Worked great. I loaded it up again this evening, and after an update, now I can't see through portals any more. They're just black, until I step halfway through, and then I frequently see what's on the other side of the wall, or a synesthestic acid trip, instead of what's on the other side of the portal, until I get almost all the way through. I tried changing the render depth, with no improvement.