A patent has been filed for it officially, and there was a rumor one was in prototype testing with the same 82mm filter size as the 24-70 II.
Personally, I believe it will definitely arrive and not too far in the future.
I think its just part of Canon's marketing strategy to sell people the 24-70 II without IS first for a couple of years, then sell them the IS version again when they are ready. There is nothing magical about the 24-70 range that makes it impractical to do IS on, but there is a financial benefit to releasing as many iterations as possible and have people continually upgrade.
Here is the patent for the Canon 24-70mm f/2.8L IS:
http://egami.blog.so-net.ne.jp/2013-09-21
Releasing innovative lenses when the time is right is what will keep Canon on top. 24-70 IS is an example of that. If you have something else that works I would hold off for the IS version, because image stabilization is *always* useful to have unless you carry a tripod/monopod with you everywhere. Necessary, no, but useful heck yeah.