Section 4.2 of the draft OAuth 2.0 protocol indicates that an authorization server can return both an access_token (which is used to authenticate one's self with a resource) as well as a refresh_token, which is used purely to create a new access_token:
http://tools.ietf.org/html/draft-ietf-oauth-v2-10#section-4.2
Why have both? Why not just make the access_token last as long as the refresh_token and not have a refresh_token?