I have a page where the user enters address data. They can select a country from a drop down list. Depending on what country is selected, the user enters their province/state by either a textbox or a drop down list.
If they select USA/Canada, a drop down list is populated with the states/provinces and made visible, while the state textbox is made invisible. If they select a country other than USA/Canada, the state drop down list is made invisible, and a state textbox is made visible. The first item in the drop down list is "select your state/province" with a value of "".
you can have 2 different validation controls to validate the same control.
You can add a compare validator to the state control. Set your "select your state/province" with a zero or a high number you are sure the rest of the options don't have then set the value to compare to that number. Set the operator to NotEqual.