It's possible they needn't worry. An awful lot of policymaking is done at sub-Cabinet level, and it's entirely likely that the less high profile appointments will be substantially to the left of those we've already seen.
What's more, if you look at the Harvard classmates with whom Obama has surrounded himself, they are true blue liberals (a couple of them, Tom Perelli and Chris Lu, are very fine people). From his history, that (and even left of it) is the milieu in which Obama seems most comfortable.
All that being said, as I argued here last week, apparently Obama understands that America is a center-right country. And he knows that the left is never, ever going really going to turn on him. To twist an old aphorism, he doesn't need to buy the left a cow, because he's almost always going to get its milk for free.