How to use Microsoft Account in an openID site?

As far as i know they still only use their "Passport"/"Windows Live ID" system, so no. They have announced however that they are going to support OpenID at one point or another. Take a look at the link below there is even a Tech Preview to test out their implementation maybe that can be used somehow:

Microsoft & OpenID

Until Microsoft supports OpenID my current plan is to hide OpenID and Windows Live ID behind a "Third Party Auth" interface. Using the Chain of Responsibility pattern I'll choose the correct provider and the rest of the app won't know the difference (except the UI where I'll have friendly buttons, much like the ones for Gmail and Yahoo accounts).