Canon has yet to announce the G13, but rumors of a fundamentally different camera have been bubbling for some time. Combined with a new statement that Canon has no interest in entering the small-sensored ILC market, and the unavoidable fervor created by both the Fuji X100 and X10, I think it highly likely that Canon is going to make a larger-sensored, fixed-lens camera of some sort.
As such, I find it reasonable to assume that something is in the works at Canon that will have a large sensor without an interchangeable lens. I doubt that they will go up to a full APS-C, not just because the optical design would be difficult, but because Canon is stupidly conservative and would not want to step on the toes of their low-end SLR cameras.
I also doubt that they will go with a sensor in the 4/3 range. While many people supposed that Nikon didn't choose that sensor size because it came too close to their SLR cameras, I think that they also wanted to avoid, for lack of a better term, legitimizing the sensors inside of Olympus and Panasonic cameras. Canon would have no issues having a similar sensor size to Nikon, and as such, I think that the new G13 would have a 1"-ish sensor.
That is, of course, if the new G13 has this sensor. I feel confident that something like this is in development, but there is no guarantee that the G13 will be that camera. Canon might wait until next Christmas to put this camera out, and a G13 would likely come before that. They might launch a new line aimed at enthusiasts similar to the X100. They may also announce the camera at Photokina and then just not release it until November. All of these options seem feasible.
All I know is that Canon better act soon. The P&S to own is currently made by Fuji, and if Fuji can use that as a bridge to get people into their upcoming ILC system, Canon could see noticeable market erosion in the high-end P&S arena.
Now is the lack of a mirrorless camera a bad idea? That's an entirely different post.