I am assuming you know how to kick the work off and just need the calls to authenticate for the user.
Your facebook app must request extended permissions from the user.
http://developers.facebook.com/docs/authentication/permissions
offline_access
Enables your application to perform
authorized requests on behalf of the
user at any time. By default, most
access tokens expire after a short
time period to ensure applications
only make requests on behalf of the
user when the are actively using the
application. This permission makes the
access token returned by our OAuth
endpoint long-lived. NOTE: If you have
requested the publish_stream
permission, you can publish content to
a user's feed at any time, without
requiring offline_access.
Once you have done this the oauth_access_token returned from Facebook is an offline_access token and can be used anytime until the user revokes your app access or extended permission.