Automatically calling the first image attached to a post as the featured image

The featured image tag is a fantastic way to embellish a category array, but as simple as it can be to attach some users are stymied by too many steps and can get confused by the fact that featured image visibility is not a default screen option. There are several good articles out there on the subject:

all of which say to insert code along these lines into the functions.php file of the active theme:
NOTE: once an image is inserted into a post, it will remain “attached” to that post whether it is deleted or not. This means that with core code you will never be able to get rid of the featured image associated with the post unless you detach the image from the post this handy plugin will get you out of that mess!

This article describes a plugin based solution.