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Apple has posted a great introduction to developing Automator Actions. Any developer can create actions for Automator, as indeed can system administrators or "power users" who are familiar with AppleScript. However application developers have a particular motivation for developing actions.
They can create actions that access the features of their applications, and then install these actions along with their applications. Users of Automator can then become aware of the applications and what they have to offer. Developers can also contribute to Automator by making their applications scriptable or by providing a programmatic interface (via a framework) that developers use to create their actions.
Apple includes a suite of ready-made actions with Automator, but developers are encouraged to contribute their own actions. You can create actions—which are implemented as loadable bundles—using either AppleScript, Objective-C, or a combination of the two languages. [d/l PDF]