Michelle,
I use a char field for things like zipcode and phone numbers. Even though the field can only contain digits you aren't ever going to do mathematical operations on them. You can use an input mask of 9 to make sure valid characters make it into the field.
>That's a good point. The zip4 field tends not to be filled in and shows up as a 0. I was wondering what to do about that. I guess I missed the obvious solution. :)