A few days ago, GitHub launched a new Commit Status API. The API lets you integrate continuous integration setups to report back the status of a build to pull requests on GitHub.
We use GitHub ourselves and have been wanting something like this for a while and we immediately implemented support for commit statuses in our existing GitHub integration. We needed to add some additional interface before making the feature available, and that is why you're only getting it now.
To take advantage of commit statuses, you may have to deactivate and re-add the GitHub integration on AppHarbor. On GitHub, deactivating is done in Admin -> Service Hooks ->AppHarbor. Once that is done, you can re-add the integration from the Settings menu for your app on AppHarbor.