It's not the SDK, but it's a release schedule, and a sign that it's not just a "Oh, we're gonna do it eventually, really.."
The other big thing I hope some of the people on the net take away from this is the three parties involved. Release plan developed by Volition, sure. Then given to Deep Silver for checking and input. Perfectly standard. But then going to the modding community? I mean, we know why (because let's face it this probably won't involve some needed tools if they're really internal designed like expected, meaning coders here have to fill in gaps using Volition provided info, and even so there's a lot to coordinate) but to those outside it's also a thing that says "They're working with the community on this one and addressing concerns."
Regardless of the number of reasons for modding sdk, it's also a good publicity thing with SR4 almost out the door, saying "We know you love this game, we love it too, so why should we hog the making of all this fun shit? Buy this game and if it doesn't include the cool thing you want, there's a good chance you'll be able to add it and show your friends too."