I just played my second game of Mac portal today, and before the game started, Steam downloaded an update.
It didn't fix any visible bugs for me, but it looks like it introduced a new one. Now when I look into the portals, all I see is black instead of the room on the other side. Yesterday when I played, I could see through portals fine.
It is just a glitch on my specific hardware, or did any other Portal Mac player get this too? I figure that if all of us are getting this bug, it'll probably be fixed soon. But if it's just me, I might be outta luck.