A press release from McDonald's (at least according to a few other blogs) says that this is just to create buzz marketing for the Quarter Pounder (not yet on menus at McDonald's in Japan) as it's introduced.

Dr. Tantillo's just published a post on McDonald's on his marketing blog, in which he praises McDonald's for being so flexible and adapting to demand but prefaced this with: "The big things about McDonald’s cosmetic image (i.e., colors, designs, clown mascot) have never essentially changed. Neither has the basic serving and restaurant setup."
Guess they are even more flexible though... Tantillo's full post

Even though this is supposed to just be temporary/buzz marketing, I wonder if cultivating this sort of aesthetic wouldn't make sense for them--a move to diversify their holdings (they own a significant portion of Chipotle, or did last I knew) and expand/diversify their customer base (Ex. their 'unsnobby coffee' campaign. Tantillo did a post on
Starbucks/Dunkin' Donuts mentioning McDonald's coffee, too.