Yep, Age of Empires Online is officially here!
I've been very disappointed with the lack of hype for this game, as I found it immensely fun. Yes, the cartoony graphics is quite a departure for the series, but the gameplay itself hasn't been dumbed down in any way, it actually plays a lot like AoE2 and the social features can be largely ignored. You can also download the F2P version, which gives you access to quite a few features for completely free.. so you have no excuse for not trying it.
But yes, it uses Games for Windows Live.