It would, indeed, be a step down, but both pay lucratively, one way or another, and do not suffer from the same term-limit restrictions. We should also probably keep in mind that a number of US Presidents took 'step-down' jobs (Congress, etc.) after they'd left office - and Obumble is too young to retire... I dunno.
No president has run for a lesser office since Andrew Johnson in 1874, and that was after two failed attempts to win election.
But if Obama were to follow that route, his best bet would be to run for Senate against Mark Kirk, who really hasn't been the same since his stroke and has alienated most of his Republican base by being, well, reasonable.
Obama could also just be waiting for President Hillary to appoint him to the Supreme Court.