According to Game Informer, you'll encounter Anders in the slums of Kirkwall, where he's traded pithiness for pity long enough to heal injured Blight refugees. Anders' motivations are unclear, but his mastery of magic should make him a valuable party member -- especially after you trade him in to the chantry for a healthy reward, we imagine.
Update: It seems Anders may have had ... a change of heart since the events of Awakening. Beware of spoilers. [Thanks, Voltron.]