I was about to respond and tell you to not worry about it, but it appears that would be bad advice.
A couple of years ago when I pulled my credit report for the first time I freaked out a little bit when I found out I had 2 credit cards on my account that I didn't know anything about. It took a little bit of time to understand that those were my parents CC's from back in HS when I was on their dime. At the time there was some big news that Fair Isaac was going to stop using accounts where you are an authorized user for calculating your credit score. Well, it turns out they changed their mind:http://www.credit.com/credit_informatio ... anges.jsp/
so it turns out i'm in the same situation as you are
and didn't even know it until just now! I've pulled my credit report since then but always glossed over those cards since I knew what they were.
Just to be clear, you are no way on the hook for paying the balance on those accounts. Your FICO is the only thing affected.
If it's any consolation I'll be calling my mom and dad as well to beg them "please take me off your credit card." Yeah.. sometimes it feels the umbilical cord is longer than you think!