We've heard that Google was testing custom homepage backgrounds, but the feature now appears to be live for all users. Yes, it's probably not really all that useful, but it can at least make your day just a bit lovelier.
You can simply head over to the Google homepage and look for the little "change background image" link in the lower left hand corner. Click it and you'll get this menu allowing you to select an image of your choice:
Photo of the bunny that I intend on kidnapping is by Jason