Joomla! Development Team so far has been following its committed Beta release schedule closely. Ever since it started Joomla! 1.6 Beta phase, every other week we receive a new Beta version. Yesterday (6/28) Joomla! 1.6 Beta 4 was put online for download.
If you are interested in trying it out, you may download it at http://joomlacode.org/gf/project/joomla/frs/?action=FrsReleaseBrowse&frs_package_id=5300.