The next version 1.1 of my Open Graph Pro WordPress Plugin will feature an API (a public method and two filter hooks) that can be utilized by theme and plugin developers who would like to add “Like” buttons to their blogs. The API also allows you to add Custom Object Types.
If you want to test-run the API go to the Open Graph Pro download page and grab the Development Version. The API is not going to change anymore. The code is also pretty much final. I’m merely working on some final UI touches and i18n/L10n of the plugin.
Code snippets for adding “Like” buttons or using Custom Object Types are part of the API documentation.