is available now. This is the first dev build for Sublime Text 3, so you won't get an auto update notification from previous builds.
3013 has atomic saves, where the file is first saved to a new file, and then renamed over the top of the existing file. Atomic saves are disabled by default, but can be enabled via the atomic_save setting.
There are a bunch of new APIs in 3013, notably on_text_command an on_window_command. These events allow plugins to be notified before a given command is run. Plugins can use them as simple notifications, or they can change the command that will be run: if, instead of returning None, an event handler returns a command name or a (command_name, args) tuple, then the returned command will be run instead of the originally issued one.
It's easy to get into trouble with on_text_command, so do be careful. If, for example, you make a plugin that unilaterally returns "paste" from on_text_command, then every command issued will be a paste command, making things like undo, editing, navigation, etc not work. You'd have to manually delete the plugin to get things make to a working state.
The API reference has been updated with all the new APIs in S3: http://www.sublimetext.com/docs/3/api_reference.html
On OS X, experimental support has been added for handling line numbers passed through the byzantine apple events system. If you are using Unity 3D, please let me know if this is working for you.