For credit card expiry dates I needed to mod the Microsoft.Phone.Controls.DatePicker to only ask for Month and Year.
This was surprisingly convoluted but thanks for reflector, achievable. The xaml for the existing date picker is a resource in the dll but you do have to do a fair amount of work to keep it happy that it doesn’t have a day part anymore.
Seems to work and we’re on track for submitting the app tomorrow! 🙂