The only thing that would threaten to dislodge Canon from their place of power is if Nikon licensed Foveon (or similar) technology, and was able to produce a true 35mm, full-res, 3 colour sensor.
I see more and more of the top shooters in my country in the professional (not press/documentary, but all others) going for top quality MFDB's. And whats started recently is the lower level professionals (social/high-street) are buying into MFDB's like the P20 and CFV instead of going the 1Ds route. I'm amazed at this as I personally dont see the cost/customer satisfaction ratio being there to support MFDB purchases in this segment, but it seems some people are doing it anyways.
The top range Canons will be threatened from MFDB's as long as the back makers will move towards lower priced/higher specd. models. Its a dream for many professionals to move into MF again, and I think many would pay premium for this in todays good photography market (at least where I live...)
The place Canon has with press, documentary, wildlife and sports shooters is undoubtedly there, and is not threatened by anyone but Nikon at the moment, ant the threat is more like [span style=\'font-size:8pt;line-height:100%\']"hey you, can i join!"[/span], but Nikon and others are playing catch up.
In the lower end of both DSLR's and consumer cam's, Canon is in the same massively competitive race as everyone else, and wont win or lose without a breakthrough and protected technology, as the market is so healthy at this point.
Also the installed top end user base for 1Ds cameras, may not quickly change out what works at the moment. But sit back with the current model and lens line they own, and build up a new MF system over time (i see guys buying the H or AFD models with only the 80, and wait long periods of time before buying their next glass, slowly building up what they need while keeping their canons warm.)
So the way I see it is that Canon won't disappear and will stay at the top end, but they are threatened in some core high-end areas where they currently are the market leaders, and things may change. The same goes for Phase in my eyes, but that was on the topic of the original poster