The effect was disabled for hardware rendering since it couldn't probe the frame buffer for occlusion. It also required a full-screen translucent polygon, which was fairly expensive at the time. The rendering cost isn't as much of an issue now, but the lack of occlusion test means that the glare effect would still happen even if the sun were blocked.
Update: I was able to duplicate the software effect with a D3D9 occlusion query. It wasn't even all that hard, though the implementation is a bit ugly. I did try to ensure that it fails gracefully on graphics cards that don't support queries. I'm definitely satisfied with the end result.
