I've been a Canon man for my whole photographic life. My influence came from my father, who used an AE-1 Program and had a number of nice lenses. At the age of 15, I bought my first SLR, an AV-1 so that I could borrow some of my Dad's kit. After 28 years, I'm pretty well "in-Canon" and I can use any of their gear intuatively. When I pick up another branded camera, I find the metering, focus and User Interface work with different assumptions and feel very alien to me. Nikon's are just wrong in my hands...a confused button layout and poorly though out ergonomics. Where as I can pick up any Canon DSLR or ef lens and I can fully use it in seconds.
I've seen a lot of photographers jumping ship, Nikon to Canon....Canon to Nikon, over the years and this seems crazy to me. One might gain a small advantage in the short term, but loses quite a lot in cost....but the differences are nearly always made up within a single camera generation. So I'd rather enjoy my kit, keep my current developed familiarity and see what the next model offers.
In the current array of cameras, I think the Canon 5DIII is the most versatile DSLR currently available from any brand. I also think that it stands alone in the market place at the moment.