Today cannon announced it will offer a modification to the 5D MK II and 7D. For a $100 fee they will ad a locking mode dial (see the announcement here). This fixes a flaw in both cameras. I can't tell you the number of times my 5D MK II has changed out of manual mode accidentally.
I don't ever remember Canon offering a modification on an existing camera and I think that this is significant for those waiting for the Canon 1ds MK IV.
OK lets face it the 5D MK II is the top of the line camera. Yes there is a 1ds MK 3, but almost every pro canon shooter I know uses the 5D MK II. It is just a better camera. Also I've found that with the sensor in the 5D MK II I am running up against the resolving power of my lenses. Really I am seeing issues with my lenses that I never saw with the 1Ds MK 3. It is one of the reasons that I have mostly gone back to primes.
What I am arriving at is that the replacement top end camera isn't going to compete with the 35mm equivalent cameras we are using now. It will be significantly different, it might even look different. It certainly will need to have new series of lenses. Also in this economy I wouldn't look for it anytime soon.