FYI, I just finished setting up a sample locations.ini that has all of the EC2 agents and locations configured (7 regions, 5 browsers in each).
This is a great example of a much more complicated locations.ini but also a great resource if you want to use the EC2 AMI’s as test agents.
http://webpagetest.googlecode.com/svn/trunk/www/webpagetest/settings/locations.ini.EC2-sample