The scipt in the post that Clinton refers to works well.
Keep in mind that Marketo is revamping its forms capabilities in Q1 with some pretty cool features including the ability to create conditional fields (like Country/State).
It would be create if you accept the enabled ISO country / State Provice fields enabled in salesforce. In fact you could implemetn them the same way. When you turn this feature on in salesforce it basically created the 2 new fields in salesforce country and state/province and you can run the txt field and the pick list fields. I'm surprised that Marketo did not do this first. Then allow the pick list fields to be drag and drop fields to forms. What ever is enabled add to the form. I like how salesforce allows you to pick each country you do buinsess in and disable the rest.