Yeah Blaubart, I originally started with CCC 9.12, applied the ExDeus registry tweaks to Win7. CCC allows me to select VA deinterlace. However, in DxVA checker, VA only allowed resolution up to 720x480. When playing 1080i, it would revert back to Adaptive, as confirmed by Cheese Slices. So I began tinkering with driver and OC settings. Since some mentioned taht OC'ing the memory allowed for VA, I tried that route. Only after OC the DDR2 to 1008Mhz (effective), would DxVA checker report VA for 1080i sources. I then ran the Cheese slices again and confirmed VA functional. So then I tried downclocking to 800Mhz stock speeds and VA was retained until next system reboot.
I have tried the Mpeg2 and H264 versions of Cheese Slices and both work with 0 frame drops using MS DTV-DVD decoder. WMC 1080i OTA also works flawlessly. GPU usage hover around 50% for Mpeg2 and 20-30% for H264. this is due to the 2600 series lacking full Mpeg2 VLC decoding in UVD. So the stream processors handle part of the work.
I don't how this would play out in XP, as most of those ppl a year ago probably used. I think most ppl back then were uninformed of VA deinterlacing without a true test, and concluded that they had it working simply because CCC allowed that option.