Thanks for the test code and data, Matt! It sounds like enough browsers are covered that error reporting is no longer a major decider between XHR vs. script tags.
Regarding iframes, be aware that some GWT users can't use them. I don't know all the reasons why, but one example reason is that iframes don't work reasonably on iPhones. So, we need to support non-iframe linkers for at least some use cases. Lex -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
