Joomla 1.5 Beta 2, dubbed the "Red Barchetta", has been launched today. From the Joomla announcement:

With today's release of Joomla! 1.5 Beta-2, we're shifting gears into turbo mode; an entirely re-factored core, internationalisation, FTP layering, next-generation API and framework that's so flexible and powerful and, of course, a designer's dream for unparalleled templating flexibility.

The difference between our current 1.0.12 and the forthcoming 1.5 Beta-2 are not easy to describe, but in essence Joomla! 1.0.x was based upon the Mambo codebase, whereas Joomla! 1.5 has been completely rewritten from scratch.

It sounds interesting and I'll test it as soon as possible. When the Release Candidates will be out I'll update the current Joomla review if there are any major changes.