I've been told that a local photography store has been told to rid themselves of the 24-70 and be very selective in ordering more. When they're told stuff like this it usually means there's something to replace it coming along.
A new 24-70 2.8L IS would be the perfect zoom lens for wedding photographers and a walkaround on the 5D MkII. It does make Nikon look silly for not putting VR in theirs. Canon will introduce this lens at the same price or less than Nikon's overpriced version.
I've used the Nikkor 24-70 on a D3 and compared it side by side with the 1D Mark III (mine focuses) and Canon's 24-70… there is no reason in the world it should cost more.
Canon will not be introducing a 18-200 to the marketplace, against the desires of many amateur photographers.
The reason for not making one is simple… to make one right, the lens would cost $2000 and up. Nikon's version isn't a very good lens and Sigma's is downright awful. Just take a peak at the price of the 28-300 f/3.5-5.6L from Canon.. $2400 or so.
Nikon's 18-200 is barely better optically that the new D60 kit lens. Canon wouldn't be able to make an $800 lens perform any better than the 18-55 IS/55-250 IS combination.