Magento’s AJAXy one page checkout is nice, but determining how well each step performs can be a little difficult. Google Analytics results will just show a customer reaching the one page checkout and not the conversion performance of each separate step… The easiest way to test changes to checkout steps is then to make changes to 1 checkout step at a time and see how that impacts the entire one page checkout performance. It can also be a good thing, forcing you to view the entire checkout process holistically. Hopefully changing Magento’s workflow for the checkout process will become a little less time-consuming in the future. There are plans for making a non-AJAX  / multiple page version, at least. Combining the shipping address and shipment method steps for instance might provide excellent, unexpected results, for instance.

Update: http://www.magentocommerce.com/extension/171/fooman-google-analytics-plus. This module tracks performance of different parts of the one page checkout!