From the Microsoft Security Research Center blog:
We are releasing the bulletin as we’ve completed the required testing and the update has achieved the appropriate quality bar for broad distribution to customers. Additionally, we’re able to confirm that, in the past few days, we’ve seen an increase in attempts to exploit the vulnerability. We firmly believe that releasing the update out of band is the best thing to do to help protect our customers.
Good news! Well done Microsoft!
Check Windows Update now if you've not already installed this update. Go on, do it now!