Sky Anytime+ does NOT currently offer any streaming HD content over broadband. The HD programmes in the Showcase section are downloaded over-air the same way they were in the original Anytime TV. This is why they can play instantly.

However, there is no reason why Sky couldn't offer HD content over broadband. Yes, it would require decent bandwidth to stream in real time, but given Sky boxes have large hard-drives there is nothing stopping it being buffered to disk first and then played back. Or it could just be downloaded overnight.

I imagine the real reason there is no HD content is to save Sky's bandwidth. But maybe this will be looked at in the future?