Canon is likely saving up for a big showing at Photokina in the fall, would not be surprised if that is one of the lenses announced.
I would also not be surprised if this is not announced, if the outdated 50/1.8 and 50/1.4 have been selling so far they will continue to do so. The 50/1.2 is a unique prestige cash cow not to be easily endangered by a faster 50mm with IS and good bokeh.
The 50/1.2 is threatened by the Sigma 50A, not the EF 50/x IS. I hope Sigma can ramp up production enough to fulfill demand for the 18-35A and the 50A, so that when they release my new 85A I won't have to wait too long to get it. I am also very interested in the presumably soon-to-be-released 50 IS. I wonder if it's going to have a silly high release price like that of its siblings, or more in line with current prices.