I've added support for the WordPress 3.5 modal media chooser while retaining backward compatibility with >=3.4.2. Anyone running 3.5 RCs or trunk that wants to test the modal--or anyone that wants to verify backward compatibility while still on 3.4.2--please download the latest version from github.
BACK UP all your data/files prior to doing this. While this is backward compatible and non-destructive, better safe than sorry.
Please reply to this thread with any issues.