Based on a Half-Life 2 forum thread called For anyone with questions as to why AA wasn't turned on in the videos..., there's a story up about anti-aliasing support on (thanks Tiscali Games and X-bit labs) quoting Valve's Gary McTaggart (second-hand) as saying: "At the present, you wouldn't want to run the game with AA enabled. . you'd see some pretty bad artifacts. We may come up with some way to work around the hardware problems in the future though so that you can use AA." A couple of further quotes offer outlook on to what degree this can be addressed on current hardware:

Drivers aren't likely to fix the problem, with the exception of the ATI 9500-9800. There's hope there for being able to use FSAA properly. You are out of luck on NVidia unless either NVidia or us come up with some clever way of solving this problem.
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The problem is specifically with multisample antialiasing on any card that uses multisample antialiasing. If you use supersampling you should be fine. Actually, this problem has existed for any game that uses light maps that are packed into a subrect (like Quake 3, etc). You will be able to turn FSAA if you like (in the control panel for your video card), but it's likely to have artifacts on triangle boundaries. We don't recommend turning multisample antialiasing on for cards that don't have centroid sampling. . .your mileage may vary.