@evildictaitor: My 1080p playback is from blu-ray discs, so, no I'm not typically doing anything else at the same time (unless WMC decides to update the guide listings or MSE decides to do a malware scan). Without hardware decoding, my HTPC would be unable to play blu-ray discs without dropping frames (I've tried it, since PowerDVD enables a lot of extra filter options if I use software decoding).
I don't see how multiple streams, adaptive streams, or DRM has anything to do with video decompression. I agree that MS should concentrate on the software side. It's up to the hardware makers to support this software with hardware.