A chat log from last night's chat with NVIDIA is up on Bjorn3D. Along the way, the proceedings point the way to a forum post called Half-Life 2 - Anti-aliasing by Valve's Gabe Newell that puts the recently unveiled "problem" with HL2 antialiasing (story) in perspective: He explains how the visual artifacts described in recent reports have been present in other games for years, and that they are showing concern because they have eliminated so many other visual artifacts as well as the amount of light variation they are using. He goes on to discuss solutions:

2) What are potential solutions?

• Support Centroid Sampling
  
• Use Pixel Shaders to Clamp Texture Coordinates

Centroid sampling doesn't have the problem that center sampling does in multi-sample antil-aliasing. ATI has supported this form of anti-aliasing for the 9000 series. The tricky part is enabling this when DirectX doesn't easily expose this.

There's a different trick you can use with hardware, such as NVIDIA's, that doesn't support centroid sampling. Basically you trade off some pixel shader bandwidth to clamp the texture coordinates so that you don't sample texels outside of that polygon's lightmap sub-rect.

Between these two approaches, multi-sample anti-aliasing artifacts should be a non-issue for any DX9-level hardware running Pixel Shader 2.0.

3) How will this look?

We'll release one of the demo movies with the anti-aliasing artifacts in and one with the anti-aliasing changes.