Ugh, that's lame. I was doing a bit of web hunting last night, and I surmised it'd be similar to a service offered by Sky UK where a selection of the week's programming was automatically downloaded to your MySky box, which I thought was pretty cool, and FREE! But hmmm, it seems not..
It's Sky's (overdue) movie on demand service. A chunk of space on the MySky HDD has been reserved for this since day 1 and the decoder downloads movies in the background so you can watch them when you want. It was due for launch months ago but has been delayed due to various reasons.