In my opinion, if Canon does not launch an cheaper model around 2500-3000€ with 35+MP, they will loose a lot of photographers.
Are they 'losing a lot of photographers' to the D800 now?
I don't know how many they've lost so far, but if this rumor turns out to be true, I'm guessing they'll lose a few more.
I'm a landscape guy, and often end up chatting with other landscape shooters in the field, for example waiting at a spot waiting for sunset. I've had about half a dozen people in the last few months tell me they were seriously interested in getting the D800, but were first waiting to see what Canon came up with. Plus I've seen the same comments from other landscape people on forums.
If this EOS-1 is the only high MP camera announced any time soon, and if it's priced as high as some of us suspect, it'd be interesting to see how many actually switch. I sure some will, especially to use with Nikon's 14-24. But some will change their mind once they factor in having to buy Nikon glass and accessories. Obviously I'm simplifying things, as there's a bunch of other factors that like ergonomics, which lenses these individuals have vs. Nikon equivalents etc.
Although it's looking like the 'Canon glass is cheaper' argument isn't as valid anymore for the top new lenses, given the latest prices (e.g., 24-70 II). It'll be interesting to see what older L-glass performs well with that kind of resolution; I've read arguments on both sides, but don't have the technical background to know which side is true.