Right now it is not a deal stopper for me, but I want to get more information and see what Canon's reaction is. There may be a firmware fix or workaround, or possibly a real simple change that could made on the camera body. This is a very recent discovery, and most of the discussion concerning it has taken place over the weekend.
Will this problem send a lot of people to Nikon or even Sony? That is unlikely, it is not a big enough problem for most shooters, though it may sway a few people who are already considering the switch for other reasons.
Will it stop people switching from Nikon to Canon? Yeah, probably a few people who don't truly need the 5D Mk II resolution or video.
Will it cost Canon some sales from people on the fence as to whether or not to spend $2700 on a body? Given the current global economy, I think that is a definite "yes." It is very hard to justify spending a lot of money right now for many people, and the 5D Mk II has lost a bit of its gloss with the black spot affair. Canon's older bodies are outstanding tools, and it may not be that difficult to wait out a generation, depending on what your needs are.
Does the current problem hurt Canon's image? Yes, but it will hurt it far worse if they are as evasive as they were with the 1D Mk III.