The AP is reporting that Microsoft's game advertising division (called Massive, Inc., not to be confused with Massively), has signed a deal with Blizzard to offer advertising on their upcoming Battle.net revamp, presumably to premiere with Starcraft II. This is apparently out-of-game advertising, i.e. the ads you'll see upon login to the service, not necessarily on in-game billboards or other nonsense like that. Battle.net is Blizzard's online service -- they've used it since way back in the Diablo days, and they've always had ads for their own products in it. But now they've contracted with this company Massive, Inc. to put other ads in there, and since they've been planning for a revamp for a while, you can probably expect to see the ads in right away when the new service launches.
How will this affect us as WoW players? It might not -- Blizzard may leave WoW on its own launcher, rather than having you go through a Battle.net launcher to sign in. On the other hand, WoW is clearly Blizzard's biggest online game, and we already know that Blizzard plans to include some WoW features (achievements, accounts) in the Battle.net revamp, so it could be that they're going to unify everything under one launcher (WoW, Starcraft II, Diablo III), in which case you'd see these ads when you sign in as well.
Which makes us wonder why exactly we'd be paying $15 a month to see more ads on login, but don't panic yet -- we'll cross that bridge when we actually know it exists (for now, Blizzard hasn't announced any official plans to change the way WoW works with Battle.net, other than the fact that one account will work across all games). We'll keep an eye out for any other new Battle.net news -- there's definitely something big in the works.