In Epitaph II: The Return, Alpha asks Tony, "Why would someone do something so horrible to that face?" He was speaking about the tech on Tony's face, but it was also a reference to season 1 when Alpha sliced up Tony's face with a knife.
Before Dollhouse, Olivia Williams had a bit part as Moira McTaggert in X-men 3. She was showing a patient who had no conscience. Xavier then brought up the ethics of placing the conscience of one person into another which is what Ms. Williams does now as Ms. Dewitt in Dollhouse.
While Joss Whedon tries using actors he hasn't worked with before, this series features seven actors who have worked in his previous shows: Eliza Dushku and Alexis Denisof appeared in Buffy the Vampire Slayer and Angel, Amy Acker appeared in Angel, Alan Tudyk appeared in Firefly, Felicia Day, who appears in the unaired episode "Epitaph One", appeared in Buffy, Mark Sheppard appeared in Firefly, and Summer Glau appeared in Angel and Firefly. In addition, none of those seven actors have appeared on all four shows.