How can I access the currently deployed Revision in a deployment callback?

Say you want to notify Hoptoad of a successful deployment. It requires the currently deployed revision to be useful. The information is there, it's just not exactly available in the way you're used to from e.g. Capistrano. The revision is stored on an attribute of the deploy resource called scm_provider. To access the current revision, you can do the following in your callback files:

current_revision = scm_provider.revision_slug

This works the same with Subversion and Git, returning the according revision number or SHA1 respectively.