I would criticize Canon much more for their churn on the D60/10D/20D (each with a new body no less) than I would their replacement of the 1Ds after two years.
Seriously. At least the D30 / D60 could fit in the same housing. Every time Canon comes out with a rev of one of these now you're looking at a big enough change to force a new housing. Wwhile there are no "low cost" options for the 1D series, you could probably re-use a housing from the 1D all the way up through now, so that's what, 4 years? In the long run it's almost cheaper (from a housing point of view) to go with the 1d.
I think Craig's pretty much nailed the improvements for the 1Ds MkII here. Basically, amything improved in the 1D MkII is going to be leveraged in the new 1Ds, including noise, buffer, AF performance, etc.
One thing Nikon has at least been consistent with is the 1.5x crop. As a Canon owner, I'm a little tired of dealing with multiple crop factors and figuring out which lenses work on which cameras.