I have no idea, however my guess is the answer is no, for two reasons.
1) Everything else provided on On Demand is basically from a TV channel so there is some control over content. YouTube is a bit of a free for all and does have some unsavoury content that Sky could not control.
2) I believe many of the videos on YouTube use Adobe Flash which is available under Windows but not other platforms (like some tablets). I assume Flash is also not available on the Sky platform so some videos on YouTube could not be shown.