Shacknews have posted a Q&A with Gearbox Software president Randy Pitchford about the PC version of Halo:
Shack: You've stated in previous articles that the pixel shader system had to be re-written for the PC. Because of this tech update, were there any new features added to the engine? Were any taken away?
Randy Pitchford: The rendering pipeline is still Halo - which is good and solid. But, we've ported it to DirectX 9 - which is great because Shawn (Green) has been able to write Pixel Shader 2.0 effects for new, high-end video cards and implement a fixed function solution for older or lower end cards.
This was necessary because we wanted to support a wide variety of hardware and because the Xbox video processor is so different from counterparts on the PC. The techniques to make the great effects you've seen in Halo on the Xbox simply weren't available until very recently on the PC and even now the techniques are accomplished in different ways (which requires lots of work).
On the highest end PC, no features are lost and some are added. Many rendering features were adapted or otherwise replaced - some of them to greater effect.