Well, we know Sky are charging VM an 'incremental wholesale charge', I wonder if the charge increments depending on what pack you're on?
Just as an example (and
I stress this is based on purely guesswork on my part):
Sky Premium HD charge (per month):
XL: £5
L: £7
M+ £10
You know what will probably happen next, don't you?
Sky will scrap their HD charge, raise the prices of their packs to compensate for the lost income and then scream from the rooftops: "HD is FREE on Sky, those guys at Virgin will charge you extra!"