ACS recently added support for OAuth 2.0 protocol. In order to show how this can be done with WIF and ACS, Microsoft posted a sample on Microsoft Connect that shows an end-to-end scenario -- meant to be as simple as possible to show the power of OAuth.
"In scenario, Contoso has a web service that exposes customer info that needs to be protected. Fabrikam has a web site and wants users to be able to view their Contoso data directly on it. The user doesn't have to log into Fabrikam site, but gets redirected to a Contoso specific site in order to login and give consent to access data on their behalf.
Contoso web service requires OAuth access tokens from ACS to be attached to incoming requests. The necessary protocol flow for Fabrikam web site (in OAuth terms -- the web server client), including redirecting the user to login and give consent, requesting access tokens from ACS, and attaching the token to outgoing requests to the service is taken care of under the covers."
The sample contains a walkthrough that describes the components in more detail.