I fear that you don't quite get that colloquialism perhaps. It's often stated when men and women live together and the man benefits from the woman as if she's already his wife, he's yet to make her his wife yet.

She cooks, she cleans, she loves him. He benefits yet doesn't have to make that final commitment.

In this case... Nokia builds new applications, they manufacturer new hardware, they even advertise on behalf of WP7 in countries that don't care about WP7 in the least bit. Nokia licenses WP7 from Microsoft for each phone sold and all Microsoft had to do was give them some money and no real commitment beyond that.

Why pay for the milk when they get the cow for free - that's the whole statement more or less.

Nothing's for truly free, I agree. But just like the man, Microsoft is enjoying Nokia for next to nothing right now... no real commitment that doesn't benefit Microsoft.