Kelly,
If you're looking for real-time updating (i.e. you want the data to pass through with the form submit), you'll need to do some javascript work, or you could use a service like ReachForce SmartForms (disclosure: We're a partner of theirs and sell their products). You can see it in action on one of our pages:
http://www.pedowitzgroup.com/solutions/revenue-marketing-cloud/marketo. When you select your company, the data is passed through into our Marketo instance.
If you can wait until the lead enters the form, you could create a smart campaign that sifts through all of the possible area codes and assigns a lead to a state. ie:
IF Area Code EQUALS 919, 910, 704, 252 -- Change Data Value "State" to "NC" (and do this for every area code).
It would be very time-intensive, but should work once you set it up.