Several efforts have been made to create a workable way of PHPUnit testing into drupal development. It goes back several years, but it's still actual. Information about PHPUnit and Drupal is heavily distributed on d.o and other sites. Here's a list (not exhausting) of discussions, projects, etc. Purpose of this list is to have a reference of what has been done, tried, etc before. Please feel free to bring in more...
Projects:
* http://drupal.org/project/drunit
* http://drupal.org/project/phpunit
Documentation:
* http://drupal.org/node/727500
Issues / Forum:
* http://drupal.org/node/483940
* http://drupal.org/node/876258
External (blog, HowTo, Twitter, etc):
* http://patshaughnessy.net/tags/drupal
* http://kristiannissen.wordpress.com/tag/phpunit/
* CiviCRM Forum: Unit Testing
* CiviCRM Testing Documentation
* Twitter (search on PHPUnit and Drupal)
* Upal - a proposed test framework for Drupal 8
* Upal for Drupal 6