Could've fooled me - can't find a mention of HD on the C4 site, unless I'm suffering from Monday morning brain in neutral syndrome.
EDIT: Just been down and checked the Sky HD box in one of our listening rooms, and yes, there it is on Channel 140. Not a huge amount of HD content though: Happy Gilmore tonight but that seems to be about it. But Mrs E will be pleased to see that Ugly Betty is in HD from this Friday!