Google Doodles has received an actual U. S. Patent, number 7,912,915. Google filed the patent application on April 30, 2001.
The patent entitled "Systems and methods for enticing users to access a web site" lists Sergey Brin as the sole inventor and references an application filed in May, 2000. The patent actually references animated GIFs in its description, so you know the Google patent lawyers have been working on this a long time.
Here's what the filing describes:
A system provides a periodically changing story line and/or a special event company logo to entice users to access a web page. For the story line, the system may receive objects that tell a story according to the story line and successively provide the objects on the web page for predetermined or random amounts of time. For the special event company logo, the system may modify a standard company logo for a special event to create a special event logo, associate one or more search terms with the special event logo, and upload the special event logo to the web page. The system may then receive a user selection of the special event logo and provide search results relating to the special event.
Reference: Patent #7,912,915