|Google's example page using photograph of Sandy Volz|
One thought that came to me is what will happen on the occasion Google will use Doodle, a mimicry of its logo to commemorate the event of the present day. If you'll notice the Google logo becomes plain white when using your own background to perhaps blend properly with the image. If on color, chances are the logo won't blend too well with customized backgrounds.
UPDATE: There may be an option to change the layout fully and according to your taste as there are examples such as a very minimal Google homepage in this post.
Check out the official Google blog post and other people's customization posted on Twitter.