This new version of HTML_AJAX was released on 06/13/2006. It is mostly a maintenance release with a few bug fixes. If you're still using 0.4.0 you might want to upgrade.
The library seems to have stabilized a lot in the last few months, so we will now concentrate our efforts towards the first beta release.
Release notes are available here: http://htmlajax.org/HTML_AJAX/release-0.4.1