Currently, ambiguities only arise for PHP files, as PhpFileLoader and AnnotationFileLoader would both claim support. Future conflicts may occur if the XML, YAML, or PHP loaders were to receive Directory and Glob loaders (as annotations have).
Since the "type" parameter is optional, loader resolution will default to awarding resolution to the first loader to claim support. A previous hack in PhpFileLoader to avoid an AnnotationFileLoader conflict was removed, so that should be the only lost backwards compatibility with this patch. Unit tests were also created for the various loader classes, although only the supports() method is being tested.
This implementation was proposed on the symfony-dev mailing list in response to Fabien's RFC for custom loader notation: http://groups.google.com/group/symfony-devs/browse_thread/thread/3104c1a9e45799d2/20fbe393c1afe088
