As a customer I can check out on one page

[queenvictoria]

Implement one page checkout using AJAX call backs to step through the commerce stages. Use the built in Drupal JSON callbacks that automatically update the "steps to complete" block area.

Then rework the default template to fit the horizontal layout.

http://demo.onestepcheckout.com/onestepcheckout/

There are people discussing these issues here.
http://drupal.org/node/841954
and
http://drupal.org/node/1175738#841954