I could answer this question by spending a day doing it, but has anyone tried using oAuth to log in a sites user?
My concern is that since usernames are not globally unique, but only unique to that org, that salesforce wouldn't have a way to know which portal to log them in on. I didn't see any parameters to scope the org/portal.
Most of the time, with sites, the answer ends up being, "it doesn't support that"