Just tried the 8.12 drivers on my system (see spec in my profile), DXVA is working on x264 encoded files (no subtitles obviously).
I'm on XP and use VMR9 so never had flickering problem caused by graphics drivers.
It's taken so long to get DXVA working I don't think I'll ever change them!
That's strange. DXVA has been working for me since 8.5 and earlier. On Vista, with EVR using MPC Video Decoder and Cyberlink decoders.