From Lions linebacker Caleb Campbell's Twitter page:
Just got activated! Headed to Dallas. Ill take that!
I'm not sure who is being let go or put on injured reserve to make room for Campbell, but I'm glad to hear he's getting a chance to play on the 53-man roster. There's no guarantee he will actually be active tomorrow, but at the very least he has apparently been promoted from the practice squad.
UPDATE: Campbell took running back Kevin Smith's spot on the roster. Smith was placed on injured reserve because of a thumb injury.