Nah, this is pretty clearly a response to Bing. In fact, I've always suspected that the pretty images on Bing's homepage are a significant part of it's growing success. Most people just don't associate with Google's clean, academic approach to design.
I think it's moreso that some users haven't signed up for a google account (there must(!) be some) because they only use the search function and that's it. Well the background will jar them out of their complacency. When they go to change it, they'll need to sign in...serving up thousands of new google account users.
This doesn't make any sense. If all these users do is search, there's no benefit in them having a Google account. Sure there's saved search functionality, the data from which can be sold to advertisers, but Google already tracks searches in cookies anyway.
If this were the goal, it would just alienate users with the background, and inconveniencing them with a forced signup for next-to-no return.
