According to this NeoGaf post, the Street Fighter II HD Beta's been posted early on Live. To get it To get it, you need to buy Wolf Of The Battlefield: Commando 3, scroll down to 'Super Street Fighter II HD Remix Beta' and get downloading. Apparently the only fighters included are Ryu and Ken.
Quick. It's not supposed to be up there until later this week, so assume it's going to be removed.