Here was Philipp's comment on the fix for this in 6.0.
by Philipp on 08.07.2009, 14:35
The bug has been spotted in adduserform.tpl
It has now been fixed. You can find a fixed version of the file in our SVN: http://collabtiv.googlecode.com/svn-his ... erform.tpl
Just replace the one in /templates/standard/ with it and you should be fine in Safari4