It seems like the consensus so far is a true "wait and see" on an HTPC-specific setup of Windows 8. There is potential for the media ecosystem to grow through Metro apps (apps for your netflix/hulu/XBMC/Plex/VUDU et al if the developers see it worthwhile or profitable.
"Media Center" in its DVR/CableCard capacity may be relegated to an even deeper niche, but hopefully another provider comes up with a way to use an app for your CableCard/Tuner and DVR needs, be it Ceton or SiliconDust or another wayward DVR software developer. But that is all unknown.
if I got out my crystal ball, the "HTPC" insofar as what we know now will have to be Windows 7 based. Windows 8 may build up the apps needed over time to become a different kind of Media Center but I would bet MS wants you to buy their new Xbox next year for all your TV/Media needs on your big TV.