I'm about to launch a Drupal site that is a compilation of custom modules and themes the company that contracted me would like to provide as a service to other companies. In the footer of the site they would like to place something similar to "powered by such and such" with a link to their parent website where they can sell additional services. I mentioned to them that the wording should be more along the lines of "powered by such and such on Drupal" or something similar. They are perfectly fine with that but now we are curious what would be the correct way of referring to Drupal so that we are in line with the GPL. I'd be interested to know if there are any guidelines or thoughts out there that should be taken into consideration before this product is finalized. Thanks!