I agree with Andrew, there wasn't any advertising for their HD channel what so ever. You'd think as HD is the latest "thing" there would have been something stating it's release. A quick visit to their website (www.channel4 .com) gives you some info.
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!