Yes, it's that special, ney magical, time of the patch cycle when content hits the background downloader to be delivered to your eagerly waiting hands. Where you'll do approximately nothing with it all, since everything being downloaded is just artwork and large assets for patch 5.2.
Could this be an indication patch 5.2 is on the horizon? Not immediately, no. This is just a good sign that Blizzard has the major things ready and is just working out the kinks on the way that the systems and mechanics work. Those kinks might take another month to work out, but hey, that's what the PTR is for.
In the mean time enjoy this 1gb download!