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.
I don't think their backgrounds are a response to Bing at all.
Somebody high up at Google (won't name any names) is a sincere, enthusiastic patron of the arts, and wants to share the gift of art with the world.
(I love art, but I think this mission is misguided, and wrong place, wrong time.)
Google has had TONS of novelty background/skin campaigns for iGoogle, Chrome, and other properties. (Often including prominent artists and designers.)
The home page is just the most recent victim.
The problem is not about quality of the art, which is usually great. It's just about it serving the user/tool, which it doesn't.
I do like the holiday/themed Google logos (pacman anybody?). But I consider those a completely different initiative.
Anyway... I repeat: It's not a Bing thing. It's a bring-art-to-the-masses thing. :/