If you write to one of the credit bureau, they have 30 days to contact the creditor to verify the debt. This will force your dentist to acknowledge the error and it will be removed from your credit report.
If you haven't done so already, check your credit at www.annualcreditreport.com, and follow the instructions at the end of the report to dispute this entry. Send the letter by certified mail can prove their receipt, and make sure to keep a copy.
I'm only licensed in CA. Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
I would first put something in writing with the dentist. He or she has told you that they will reverse this false charge. Get them to put this in writing on their letterhead. If they do not, then you need to put it in writing in a letter by certified mail to them. Something such as, "This is to confirm our conversation on May x, 2011 in which you told me that the charge of $720 to my account was incorrect and that you intend to reverse it and withdraw the collection from the X debt collector. If this is not correct or if you are not authorized to reverse the charge, please contact me in writing within 10 days."
Again, please be sure to keep a copy of this letter with your proof of delivery by certified mail. My web site has several sample letters that you may want to use as guidelines to help you dispute this bill with any debt collector or with any credit reporting agency.
Robert Stempler (please see DISCLAIMER below)
NOTICE: If you do not have a signed attorney-client fee agreement with the Consumer Law Office of Robert Stempler, APLC ("the Firm"), then until such written fee agreement is provided and signed by both client and attorney for a particular case, neither Mr. Stempler nor the Firm will represent you nor will they be your attorney in any matter and you remain responsible for any and all deadlines and for any statutes of limitations that may pertain to potential claims. Nevertheless, communications with Mr. Stempler by a prospective client remain confidential and are considered privileged.