Canon’s lineup of lenses currently includes both a 24-70mm f4 L IS and a 24-70mm f2.8 L USM–but no f2.8 version with IS built in. There were rumors being passed about this for a while now due to a patent, but Canon Rumors and Photo Rumors are saying that it’s a very strong possibility that we may see one in the near future. With Tamron, Tokina and Nikon all having their own version, it just makes sense.
According to Canon Rumors, a couple of prototypes exist and if the company is going to try to develop a lens to resolve their 120MP or 200+ MP prototype camera, then they’re going to need better glass. It’s also stated that this lens would perform better than anything else in the Canon lineup and also offer the new BR element to keep down fringing. The BR (Blue refractive) element is present in the company’s 35mm f1.4 L II.
If we see something like a Canon 24-70mm f2.8 L IS come to market, I highly doubt it will be this year and that Canon would instead save it for next year considering that it is a Photokina year.