Here's a list (not exhausting) of discussions, projects, etc about this topic on drupal.org and other websites.
- http://drupal.org/project/drunit
- http://drupal.org/project/phpunit
- http://drupal.org/node/483940
- http://patshaughnessy.net/tags/drupal
Purpose of this list is to have a reference of what has been done, tried, etc before.
Please feel free to bring in links, not mentioned yet.