ado.net data services - how to grab subsets of data from the data model?

Question

Hi, Im starting to use ado.net data services in a little silverlight application that Im putting together. Currently I'm using it in the most simplistic was possible:

var teams = (from t in ents.tblTeams
                         select t) as DataServiceQuery<tblTeams>;
            teams.BeginExecute(
                (ar) => this.cmTeams.ItemsSource = teams.EndExecute(ar).ToList(), null);

This is all well and good but I would really like to be able to grab only a select set of data from the database. Currently, I'm grabbing two full tables but I only need usernames and id (I may need more later).

I had thought that maybe id be able to perform this shaping within the silverlight by selecting only a subset of data:

var teams = (from t in ents.tblTeams.Expand("tblUsers")
                     select new TeamDTO
                     {
                         TeamID = t.TeamID,
                         TeamName = t.TeamName,
                         Members =  (from u in t.tblUsers
                                    select new UserDTO { UserID = u.UserID, UserName = u.UserName }).ToList()
                     }) as DataServiceQuery<TeamDTO>;

        teams.BeginExecute(
                (ar) => this.cmTeams.ItemsSource = teams.EndExecute(ar).ToList(), null);

However, this code doesn't seem to work (Object reference not set to an instance of an object - probably my bad code).

Basically, is there anyway to return only sets of data or is that something that I have to build in at the server level, with the data model somehow?

Thanks,