I'd be suprised if Canon has not built many prototype sensors of various mp ratings. They are said to be actively pursuing the rear lit technology too, so those will exist.
However, I expect that they are prototypes, you don't go into production on multiple sensors unless you are committed to using them, since production startup costs are very high.
@Mt Spokane: Talking rear lit sensors, I guess within three to four years time (1Dx, 5D3 life cycle) we might probably see some (r)evolutionary improvements then. Or will the first outing of such a kind of sensor be presented in a Rebel? The high MP count body might offer an attractive platform as well. So, if I am not totally wrong, high ISO IQ could improve more until 2015. Not higher ISOs, but maybe 51k ISOs equivalent to 25 or 12k ISOs. What do you think?