Well, that wasn't fun while it lasted. Google's Bing-like default background images that were first introduced in the wee hours of the morning have already been taken down. You'll still be able to add a custom background if you really, really want to (check the lower left-hand corner of the page) but the rest of us can get back to enjoying our minimalism.
There's no official word on why the quick abandonment of ship, but I'm guessing it had something to do with this:
UDPATE: The Google Blog has shed some light on the subject:
Last week, we launched the ability to set an image of users' choosing as the background for the Google homepage. Today, we ran a special "doodle" that showcased this functionality by featuring a series of images as the background for our homepage. We had planned to run an explanation of 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.