J.,
I don't have a teenager, BUT my youngest sister (Tori) is 15. Wow, what a fun age for my mom to be dealing with! Seriously, although not acceptable and frustrating, it's normal. My 2nd sister and I are 34 and 32 and cannot believe the things that Tori sill say to my mom! We cringe, thinking for sure that there will be the wrath of Mom coming out. My mom has mellowed out a LOT since we were younger (she's not 53 instead of mid-30's) and allows things that there was NO WAY we could have done. Sometimes, we will turn to her and say, "Tori...who are you talking to? Because there's no way you talk to Mom that way." Tori respects my sister and I enough that if we correct her, it's not like it comes from our mom.
As for how to deal with it, that's hard as it depends on your own parenting style. What my mom has started doing when Tori mouths off is to immediately end the conversation. Usually, Tori gets snippy when she's not getting her way or wants to do something and isn't getting the response she would like. So, my mom just tells her, "That's it, this conversation is over. My answer is 'No' and due to you talking to me like that, I won't discuss this further."
If it's something more like a snotty comment under the breath or back-talk, you could maybe respond with something like, "Well, it's obvious you aren't able to carry on an adult conversation and feel the need to act like a child. One more comment and I will treat you that way and you will lose (insert teenaged privilege here - internet, cell phone, iPod, whatever).
I think that a matter-of-fact tone of voice is a good solution as it doesn't let the teen feel that their back-talk has agitated you.
Just my opinion as I still have about 12 years until we have to deal with this in OUR house.
Good luck.
T.