From their blog: http://googleblog.blogspot.com/2010/06/art-of-homepage.html

Update June 10, 11:31AM: Last week, we launched the ability to set an image ofusers’ choosing as the background for the Google homepage. Today, we ran a special “doodle” that showcased this functionality by featuring a series ofimages as the background for our homepage. We had planned to run an explanationof the showcase alongside it—in the form of a link on our homepage. Due to a bug, the explanatory link did not appear for most users. As a result, many people thought we had permanently changed our homepage, so we decided to stop today’s series early. We appreciate your feedback and patience as we experiment and iterate.