My charger for my MacBook recently stopped working.
If I took it down to the Apple Store, would they most likely replace it with a new one?
My MacBook is still under warranty.
That said, I think it's wise to bring your MB as well as the charger to see if it's the charger that's faulty, the battery, or perhaps even something wrong w/ the MB itself.
They'll replace it because your computer is under warranty, but as it has been said before, make sure you bring your computer so they could check it wasn't the charger and they can make sure your computer is ok.