After upgrading GAFYD to the new infrastructure, I was no longer able to use the Android Market to purchase Apps for my DROID. After digging around for weeks for a solution to this, I was able to glean the following solution & decided to sum it up here.
Google Apps Control Panel => Organization & Users
On the tabs below, click on the "Services" tab
Here's a direct link, if you substitute your domain name:
Scroll down in the list to the "Other Google Services" section & make sure Checkout is "switched to on".