Joomla 1.6 adds two significant enhancements. The first is a revamp of the way that extensions are handled by the system, allowing more flexibility and customized builds. The second is an overhaul of access controls, which promises to give administrators much more granular control over user rights. While trying to estimate a production launch date based on the timing of an alpha is a fool's game, I think it's fair to say were looking at least two or three months before Joomla 1.6 is ready for prime time.
While on the topic of Joomla, I wanted to point out the excellent Joomla Developer's Toolbox piece from Smashing Magazine earlier this month. It's a detailed, screenshot-packed run-through on how to get started, with links to tutorials and resources, information on how to find and use templates and extensions, and links to real-world examples of inventive implementations. It's a great article and good place to start for any Joomla newcomer.
Joomla already is one of the most popular open source content management systems available, and for good reason. It's flexible, extensible, has solid documentation, and a very active user community ready to help their peers tackle just about any issue. The solid road map and wealth of resources are sure to bolster this even more.