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Romita has worked for Marvel for almost his entire career, with a handful of exceptions, and has never before worked for DC. His father was also a Marvel lifer.
This is apparently a long term move; DC co-publisher Dan Didio described Romita’s work on Superman as "hopefully just the first of many characters" he’d be drawing at DC.
Romita will be working with a long-time collaborator: Klaus Janson will be inking his work on Superman, according to Comic Book Resources.