I noticed that several moderators have just changed their avatars in a similar style:
Is there any backstory behind this?
...and yeah... that's basically it.
You are a fool for asking such a question. Do you realize what you are getting yourself into?! The magnitude of what you've uncovered... the extent... it is beyond your comprehension. It is an apparatus bigger than any one man, any one country, or any one motivation.
It is the Invisible Hand.
It is Majestic 13.
Now, you may notice that I have a similar iconograph for my gravatar as well. Do not let this concern you. We are not ready to be introduced yet. I suggest you put your nose to the pavement and do your job. If you keep digging this hole, you might find yourself buried. Turn back, for your own safety, and for the safety of all your loved ones.
Actually, now that I've revealed our secret society to you, I'm afraid I'm going to have to activate your killswitch. Another fifty billion dollars down the drain.
(Tongue in cheek vague Deus Ex reference; if you take this as serious, you have a problem.)
You see, SE is planning to restrict avatars to white text over a coloured background in the Microsoft® Windows® 8 Metro® style. While they are trying to get permission from Microsoft® (don't want to be sued for trademark violations!), we're just testing to see what it would look like.
Don't worry, soon you'll all be able to do this!
I'm just joking
As part of the upcoming release of a new official Stack Exchange Metro app for Windos Phone and Windows 8, a new feature detects your OS and adjusts the user avatars according to the official Microsoft design guidelines for Metro. It is being rolled out gradually, because all avatars are changed on-the-fly, manually, using underpaid workers from overseas outsourcing.
Or maybe we're all just a little bit silly.