I wouldn't bet on Canon releasing a 24-70mm f/2.8 IS, but there's some logic in it.
It seems some people like Tamron's stabilized 24-70mm for video, Sigma is rumored to release one as well, and Canon wouldn't bother testing a 24-70mm f/2.8 IS if didn't think there's money in it to begin with, so it might release one so as not to lose sales to the competition.
[Canon already made an investment in R&D, so it already has something to lose. It might make some kind of compromise to keep the price down and/or release a cinema version to cover some of the R&D price in that market to keep the lens profitable.]
Canon has four versions of the 70-200mm, so why not three versions of the 24-70mm?