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.
- http://drupal.org/project/drunit
- http://drupal.org/project/phpunit
- http://drupal.org/node/483940
- http://patshaughnessy.net/tags/drupal
Please feel free to bring in more...