Vista is about to get HDTV and DVB-S/S2 support and possibly Blu ray playback, from inside the OS itself.
Before the end of the year, a large update to Vista's Media Center (codename Fiji) should appear. The update is already out to beta testers and a public release candidate (RC) may be available by September 2008.
There have been 3rd party plugins and hacks for Media Center available for a while that would allow Blu ray discs to be played on 32bit versions of Vista Media Center however, these hacks would not work for the 64bit versions of Vista. Media Center in Vista 64 is a native 64 bit application and, the problem with that is, it requires native 64 bit plugins and codecs.
There were also work around to allow Media Center to work with DVB S/S2 TV cards but not with HDTV channels but, now it's supported directly within the new Media Center update.
Vista also has its own enhanced video rendering engine (EVR) and HD picture quality is simply stunning.
Unfortunately it's difficult to show the picture quality in a screenshot on the forum, but here is a screenshot of BBC HD, on Vista Home Premium 64 bit (fiji update), taken a few minutes ago.