I've been asked to clarify where you can find the Virgin Media HD on demand programming as Virgin seem to be doing their darnest to hide it, so here's where you go (at least in the ex-NTL areas):
Press the Home button to access the On Demand Home menu.
Under 3. Movies On Demand there's no specific HD category, but the HD versions of around a dozen of the Filmflex films are listed in 7. All Movies A-Z next to the SD versions. They are normally £4 a pop, 5op more than the SD ones.
I can't be fussed to go through all the Music On Demand listings, but there may be some HD content in there; I've never noticed it though.
in 5. TV Choice On Demand select 9. More>. Under 1. High Definition you'll find Lost Seasons 1 and most of 2, some episodes of Grey's Anatomy and the first seasons of Criminal Minds and Day Break. For some reason, most are currently listed twice but they'll sort this out. The same programmes are listed in 2. A-Z alongside their SD equivalents (where they exist) plus there's some documentary programming like Mega Movers and Dog Fights (full details here).
Finally, there's the second listing of HD under 6. More On Demand in the Home menu, option 2. High Definition. This is where you'll find five pages of the BBC HD stuff (which gets changed pretty regularly, albeit subtly), the FilmFlex HD stuff all together, the lone Music On Demand offering and some 4OD Pay TV in the form of Brothers & Sisters.
Hope that helps. Virgin say they're adding more each week (and so far they are good to their word, though we all want MORE!). Still no word on the Channel 4 HD simulcast which is now imminent on Sky HD, nor any Channel 4 HD on demand.