There's a lot of discussion in a Digital Spy thread about the use of MPEG2 for HD transmission on Virgin Media rather than the more efficient compression technology of MPEG4 (thread here), but here's a thought: When the V+ launched earlier this year, it wasn't marketed as an HD box, nor did VM promote the (sparse) HD content available. And they're still not pushing it. Not surprising given that they have so little and they're doing their best to hide the VoD HD content they've got (see previous posts).
So, pure speculation this, could they be considering a fuller HD service launch using MPEG4 boxes sometime soon? Then they could 'upgrade' the boxes of those seeking HD content, maybe with one of the new generation Scientific Atlanta boxes charging similarly to Sky a £10 premium for the service, whilst the V+ becomes the middle tier box for customers on the XL package, lower tier getting the Samsung boxes. They could keep BBC HD on MPEG 2 so everyone gets a taster, but only those subscribing to the new box would get the additional MPEG 4 linear channels.
Just a thought...a man can hope, can't he?