Magento

Can I have a customer select a referrer during checkout or registration. I’ve seen that with other cart software; that a user can select: “Where did you hear from us: Google"… and make that field required.

You might want to rethink that. Why add one more (required?) option to the checkout process?

The referrer is already stored in the session info anyways so it should be simple enough to attach it to the order.

For most web shop owners with returning customers, it is probably better to have the “Where did you hear from us” in the customer registration section (as a text field or dropdown).

I would not slow up the checkout process uneccesary, and if the field “Where did you hear from us” should be made required, it should not be asked (or required) a returning customer. - Finally, if you don’t need the information for any useful marketing research, don’t ask.

Maybe we can hope for that magento comes with some easy configurable input fields for the customer registration section in the version 1.1.