Better solutions do exist.
It is possible, although not cheap, to get a Sky+ box modified to output a digital picture in SDI format (go to
http://www.cellarcinemas.com/cgi-bin/store/sdi.html for a description of SDI), but, as always, there are some complications.
First off, only v1 Sky+ boxes (Pace 3000) can be modified, not the current v2 boxes (Pace 3100), and getting hold of these is difficult.
Second, the modification costs about £300. Places I'm aware of that do the mod are
http://www.pluggedin.tv and
http://www.pjhifi.co.uk.
Thirdly, you need a card for your PCI with SDI inputs. I can think of three - the SweetSpot SDI (about £150), the Silk-200 (about £320) and the Immersive Holo3DGraph II (about £510).
And finally, software support - all of the above cards are designed to be used with DScaler, and don't have general-purpose WDM drivers - so won't work with Media Center. There is a reason for this - DScaler includes excellent deinterlacing and scaling capabilities which significantly improve the picture quality, and if you're prepared to spend £500+ on mods and cards you're probably going to want to see the benefits; using MC as a viewer wouldn't really achieve this.
I'm speculating here, but it's probably possible to use the above to capture the digital stream from a Sky+ box, and then use MC to view the stuff if you wanted to, but you'd be losing a chunk of the benefit.