In trying to find a way to notify developers of D7 API changes, the best solution we've come up with is to formally publish to the Drupal Development Mailing List. API change notifications will occasionally be posted here with a title starting with "Drupal 7 API Change notification"

Recently the way forms use #builder_function was changed significantly, and in many cases it should no longer be called.  (issue impact summary). If your module does anything with #builder_function, you'll want to read this carefully.

An additional note: If you are chasing HEAD in D7, you should subscribe to all issues of the HEAD2HEAD module, which is the best indicator of when your current D7 install is going to be broken by a database change.

Thanks,
-Randy