subquery in linq

from m in Messages
where m.RoomID == Convert.ToInt32("1") && ((m.ToUserID == 2 && m.ToUserID != null) || (m.UserID == 2 && m.ToUserID != null)) && m.TimeStamp > Convert.ToDateTime("03/06/2011").AddMilliseconds(999.00)
orderby m.TimeStamp, m.UserID, m.ToUserID
select new { m.Text,m.UserID,m.ToUserID,m.TimeStamp,
fromusername = Users.Where(u => u.UserID == m.UserID).Single().Username,
tousername = Users.Where(u=> m.ToUserID == u.UserID).Single().Username
}