This problem happens far more often than it should. Technically, you have a right to the overcharge for the common areas. The problem is that proving the amount of that overcharge and thus how much your landlord "owes" you is an enormous headache. The utilities almost never want to cooperate and an expert is way too expensive. If your landlord "added" the common areas to your meter after your tenancy began, he/she may face serious consequences from the utility provider.
Usually the best thing to do is tell your landlord you know that you're paying for the common areas, and ask for a proportional reduction in the rent (something relatively minor, say $10, $20).
I agree with what Mr. Major has said, but wish to point out that you ought to check your lease. It's possible that you agreed to this - whether you realized it at the time or not.
NOTE: (1) I may be guessing and/or not even licensed in your state; (2) We have not established an attorney-client relationship; (3) Sometimes you get what you pay for; and (4) If you want to send me a gift, my favorite color is orange.
More likely than not it will not be a problem for the landlord, since the common areas - typically in smaller two or three apartment buildings, must be connected to *some* account. If the landlord deliberately switched after the tenancy without your consent, you may be able to proceed, but as my colleagues said this will be expensive. That said, also please be sure to pay your rent on time and to uphold your end of the lease (i.e. clean apt and common areas, no noise, no abuse, no habitual late payment, etc). Also, if the landlord has the right to increase your rent, but has not done so, this may be something to think about. He or she can agree to deduct 10 to 20 from your rent per month, but after raising the rent by 50 per month (assuming the lease permits this or if you are a month to month tenant). Good luck.