You are within your return period. In addition to the 0.1GHz base speed (which is actually 0.2GHz in Turbo Boost), you would also get the lower price. I'd see if you can get the price differential back if you don't want to go through the hassle of getting a new machine.
I am in the situation and called the apple store. The representative only offered me 30$ refund. I am still yet to decide whether to return it or accept the rather small refund!