I read when the game first came out that Dogmeat didn't count as a companion, so you cold have a companion plus Dogmeat. I can now confirm that if you take on a companion, that Dogmeat has to leave. I'm running the Beta patch, so perhaps this is just now being implemented. Does anyone know if this change means that the Lone Wanderer perk no longer applies if you have Dogmeat as a companion?