Well, hmm.... I guess that depends on what you mean by a "perennial". To me, it is a herbaceous or shrubby plant that (given proper conditions) persists in the garden indefinitely.
Dahlias are tubers, so where exactly does that place them? They aren't herbaceous (or are they?), they aren't shrubby, and I don't think that you can call them bulbs, either. (In fact, in the garden centers here, very short bedding dahlias are sold as annuals.)
As for persisting in the garden, in the colder climates, dahlias certainly wouldn't. Even here in Zone 9b, survival is a matter of where they are during the winter. The relative few that I have grown have (thus far) overwintered in the ground (unless the ground was saturated with water), but definitely not in pots. (Ask me how I know.)
Maybe I will go pose the question on the perennials forum...
