So, it finally dawned on me that they reformed Cullen. In DA:O you either don't hear much about him, or (if you play a female mage) hear that he goes off the deep end if your hero dies or is an iron fist around the mages necks in the Mage Tower if you sided with the Templars.
In DA2 he's one of the (few...) voices of reason. He was rather adorable (if you could ignore the psychotic) in DAO; he's rather devestating in DA2.
I can't help but think this is some sort of fan service. He WAS a fan favorite.
Of course, he might have been a dev favorite, too. ;P
His somewhat large role in DA2 does explain why Greg Ellis didn't do the voice of Anders for DA2, though. Guess it was easier to explain that Anders voice was different rather than Cullen's.
That and one might not think it was the same Cullen with a different voice actor. And they wanted us to question Anders' presence.