I don't see it as a sign of Steam on Live. MS don't want that and I'm sure they'd have given in already if they did. It's simply a sign that they are acknowledging Steam is stronger on windows and if they want their titles to sell they need to be on it.
I think this is more about trying to make sure Fable 3 sells well than Live or Steam.
If Fable 3 wasn't on Steam it would lose a lot of sales IMHO, and MS are simply acknowledging that.
But Live is another matter and I don't see MS feeling like they have to allow Steam onto the servive.
Try to be reasonable... its easier than you think...







