[JWait]

Just out of curiosity how do you keep track of your customers and their orders?

While consumers may or may not want to create an account, x-cart will create one anyway. It is called "anonymous checkout". Generally, even if a customer only places 1 order they want to know the status of that order (I do anyway), and your method does not provide that, other than any emails that are sent. Also, hiding the "shipping address" section does not allow your customer to have the order sent to anywhere except the billing address. Maybe you only want to do it this way, but it doesn't allow the customer any option.

I think a better option would be to hide the "register" button but display "login" on your main page and simply allow "anonymous checkout" for those customers that do not want to create a account. You can also choose "moderated membership" in admin so that resellers can apply for "wholesale" membership.

To answer your original question, you could enclose the username and password section of the register page in a "if" statement checking if there was something in the cart, and if there is not display the username and password fields. As I said, I wouldn't do this.