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. :/