If you read the symfony devs mailing-list, you know that we still miss one big feature before being able to release symfony 1.1.0.
This is not the case anymore. symfony 1.1 now comes with a brand new configuration system. All config.php files are gone and are replaced with configuration classes.
The new system brings more configurability and flexibility to your projects.
If you already use symfony 1.1 for some of your projects, but don't have time to test the new system, you can switch to the RELEASE_1_1_0_BETA1 tag.
If you want to give it a try, read the UPGRADE file for instructions on how to upgrade your projects. The UPGRADE file also provide more information on the internal changes.
As the upgrade process remove some files and introduce new ones, reverting to BETA1 is not straightforward.
Even if all the 7700 tests pass, you might find some bugs or some weird behaviors; post your issues to the symfony users mailing list, and I will try to answer questions and fix things as quickly as possible.