It is a common misconception about HtmlUnit to think that it doesn’t support AJAX. AJAX per itself (ie XMLHttpRequest) is supported since a long time. What is more tricky is to support all the advanced JavaScript functionalities used by the so-called AJAX libraries. To ensure that HtmlUnit correctly emulates the browsers’ behaviour (currently Firefox 2, Internet Explorer 6 & 7) we have integrated the test suites or demo applications of different AJAX libraries in HtmlUnit build and many of them already pass correctly.
GWT 1.4 has been long a source of problems but, as announced by HtmlUnit committer Ahmed Ashour last week, this is now past story: HtmlUnit is now able to work correctly with the example applications of GWT 1.4!
Stay tuned, HtmlUnit 2.0 will be released soon.