Original contribution is located here:
http://addons.oscommerce.com/info/6646
I don't suggest to use it on production sites because I see that 1.03 version is not final:
I had an autoinstaller for 1.02 and a new account was created when the password field was entered on the checkout.php page.
I have upgraded to 1.03 and see that the password filed is present but a new account is not created if you fill it.
I have changed only one line in the:
includes/classes/onepage_checkout.php
to fix the checking if the Gift Voucher Contribution is installed
Legend: 
Download

Report
for One Page Checkout Version 1.06
This script is for testing purposes only!
As I see 1.04 works better than previous 1.03 version.
But if you try to edit billing and shipping addresses together you will get the same address for both in email.
I have changed again only one line in the:
includes/classes/onepage_checkout.php
to fix the checking if the Gift Voucher Contribution is installed
The Autoinstaller script requires PHP5
Original contribution is located here:
http://addons.oscommerce.com/info/6646
I don't suggest to use it on production sites because I see that 1.03 version is not final:
I had an autoinstaller for 1.02 and a new account was created when the password field was entered on the checkout.php page.
I have upgraded to 1.03 and see that the password filed is present but a new account is not created if you fill it.
I have changed only one line in the:
includes/classes/onepage_checkout.php
to fix the checking if the Gift Voucher Contribution is installed