Canon Rumors reports a rumor saying that Canon is testing prototypes of an EF 24-70mm lens, supposedly named EF 24-70 f/2.8L IS. Quoting CR:
[...] received two separate communications in regards to an EF 24-70 f/2.8L IS being in the wild in prototype form. We knew this prototype has existed for quite some time, however we figured the concept was dead with the release of the EF 24-70 f/2.8L II. Canon would bring this lens to market alongside the non-IS version. Canon already does this with the 70-200 f/2.8s & 70-200 f/4s, so it’s possible they bring it to the standard zooms as well.
None of my sources confirmed this rumor to me. However, according to CR an image stabilized version of this lens definitely exists.