Accessing Current User In Portal Service

Regarding Portal Services:

I would like info on how Portal Service requests are processed. I am interested in

performing operations for the current user within a portal service. If I pass

IUser reference to the portal service can it perform operations like PCDirectory

lookups for the current user. So each user would pass a reference to its IUser object to the portal service. I notice that the portal service seems to have no access to the current user or session, so I need to know if they are suitable to perform this type of work on behalf of the current user if the current user is passed to them from a portal component or a WD component?