As Fabian Potencier has posted in the Symfony’s blog, Symfony2 is not ready yet. The core team of Symfony has decided to delay the release of the Symfony2 final version.
The core architecture is now stable, except for a few changes in the naming conventions that will be done in the coming days.
Upgrading from one preview release to the next one is now easier, thanks to the new UPDATE document.
Also, the first beta is not out there because the core team is waiting for the last big modification that will possibly be merged: the form framework refactoring. Entering beta means that all the main features of the framework will be available.
So, looking forward for the beta release at least.