As the holder of the Drupal trademark, I've recently decided to modify the footer on drupal.org to state the fact clearly by including the wording "Drupal is a Registered Trademark of Dries Buytaert"; this is considered a common trademark practice and has been advised to me by my lawyers to clear any potential confusion.
Trademarks are important because they help prevent confusion by distinguishing one company's, community's or person's work from the products and services of another company, community or person. A trademark essentially serves as a badge of origin and is not to be confused with copyright, ownership or licenses.
Together with the Drupal Association I'm working on a permissive formal trademark policy modeled after Ubuntu's trademark policy. I'm sure you'll find it reasonable. As soon the trademark policy is available, we'll link to it from the footer on drupal.org.
Rest assured that this change is intended only to prevent the 'Drupal' name from being used out of context by overly aggressive entities. Most of you who use Drupal, commercially or otherwise, need not worry. This change will only help ensure that the effort of all the hard working Drupal contributors isn't misappropriated.
Until then, stay tuned, and enjoy using Drupal!
— Dries Buytaert