Thanks again Bob.
I just implemented a slightly different version of this idea on a client site using your instructions. Seems to have worked like a charm!
I did away with the separate Customer Details area and combined the Contact Details area with the Billing Info area. I left the Shipping Firstname/Lastname in place to allow for gift orders to be addressed to person other than the purchasing customer.
Here's a screenshot.
http://www.xcartmods.com/screenshots/checkout1.gif