They're allies in some areas: The promotion of Pay-TV and their desire to weaken Freeview and by proxy, YouView. Given a choice, its better for Sky to get someone on VM (
especially on the Sky Premium channels - even more so on Premium HD content), instead of Freeview.
VM is also one of Sky's biggest customers (probably THE biggest customer): 3.7 million potential customers on digital cable and rising every quarter. VM give Sky carriage/wholesale payments for dozens of Sky channels; HD content, and soon, Sky VOD content and Red button.
Look what happened when Sky ****ed off their biggest customer over the Sky Basic dispute - Sky 1 lost 25% of its audience overnight; ****ed off advertisers; and didn't get grab anywhere near enough Virgin customers to make up for the shortfall of cash.
So, for TV, they have a common enemy: Freeview. And the enemy of their enemy, is their friend. Where its in their best interests to work together and have lunch together, they will.
Of course, after they leave the lunch table, they will still attempt to one-up each other on aspects of their TV service:
- Sky will push their extra HD channels (less so now then a year ago - since then we've had The Year of HD on Digital Cable), 3D content, and Push VOD Service, now they've got one.
- Virgin will push their 'free' HD on XL; Superior VOD Service with superior delivery to the home, and of course, the potential game changer for Cable: TiVo, which has the potential to make Sky's HD boxes look like Sinclair ZX81s by comparison.
As for broadband - completely different story. Adversaries, definitely. And we all know that VM whips Sky's arse for that. Fibre-optic vs ADSL delivered down ancient copper lines - no contest!

And the arrival of TiVo will help underline that.
There's no real emotion involved for either company as far as the 'relationship' is concerned - its just business and like any business they will do what they have to do make money.