We've heard that Google was testing custom homepage backgrounds, but the feature is now live. Yes, it's probably not really all that useful and simply imitating Bing's backgrounds, but at least it can make your day a bit lovelier.
To check out the feature, 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