We released Doctrine 2.1 Release Candidate 3 after some important changes to the new annotation reader. That is also why we didnt manage to keep our release date of June 30th. The new date is Monday.
About the Annotations: We constantly had problems with the autoloading of annotations through PHP autoloaders, increasing with Symfony2 starting to use the reader with enabled autoloading. The problem with reusing the PHP autoloader can be explained easily: Not every Docblock Annotation has a corresponding PHP class that can be instantiated by Doctrines AnnotationReader. The question is how we detect the annotations from the documentation related comments only. The solution required a silent autoloader, meaning no failure should occur if an autoload is triggered for a non-existant class. Implementing such an autoloader has performance drawbacks though, which is why the Doctrine does not fail silently. Autoloading annotations was not compatible with Doctrines own class loader.
That problem is why we removed PHP autoloading completely and reimplemented our own autoloading mechanism inside Doctrine Annotations. Now we have much more control about when errors occur and if we skip them or throw an exception.
See this following Gist on how autoloading annotations works. This is not relevant if you use \$config->newDefaultAnnotationDriver() with the ORM though, which will handle all the necessary bootstrapping for you.
Additionally we have changed the annotation reader in other aspects. To be a valid annotation your class have to extend (backwards compatible way, will be removed in 3.0.x) or contain @Annotation inside the class level docblock.
We will update the documentation for Doctrine Common accordingly in the next hours to allow you to understand all the changes made between 2.0 and 2.1.
If you are using Annotations either in Doctrine 2 or within your own code please update and check the changes.