The current version of script.aculo.us is V1.8.0.
The current in-development version is designated as V2.0. Check the Development page, and get the SVN trunk to see what this is about… :)
The next major release version of script.aculo.us is V2.0 at this moment (6 February 2008).
To get the very latest version (and maybe submit an patch, hint, hint!), point your SVN to:
$ svn co http://dev.rubyonrails.org/svn/rails/spinoffs/scriptaculous
You can also browse the repository on the Ruby on Rails Trac.