I doubt I will change to Nikon... too much invested in Canon equipment - however I also doubt I will buy the 5DmkIII. I am quite happy with the mkII right now and not seeing enough of an improvement to warrant moving up yet. Just thinking about what I would do for my next camera and why the Nikon is cheaper then the Canon....
IMHO, the D800 vs. the 5DIII debate is easy. Do you like the actual IQ of the D800 better than the 5DIII, or are you merely impressed by its stats? If you like the IQ of the D800 better, and need the extra resolution, switch systems. If you prefer the look of the 5DIII's files, and 22 megapixels are sufficient for your needs, stick with Canon. I had both bodies on pre-order, but as the sample images from both bodies started trickling out, I grew less impressed with the D800. So, I cancelled my D800 order and my 5DIII will be here tomorrow
Furthermore, is there a rule that says you must upgrade bodies whenever a new one comes out
? If I had a 5DII, and didn't need an vastly improved AF system and faster burst rate, I don't know that it would be worth it up upgrade to the 5DIII. I went through the same dilemma with then 5DII came out, and ultimately decided to stick with my 5DC until Canon addressed what I felt were its most glaring weakness: AF and FPS.
IMHO, there's absolutely nothing wrong with older gear. If anything, older gear amplifies your mistakes, opposed to hiding them, and forces you to improve your technique and become a better photographer. I have several pro buddies of mine that have been using their 1DsII's since 2005, and are still producing phenomenal images with them. The grips on them are worn smooth at this point, the but bodies are still alive and kickin'. It's not like a camera suddenly stops producing great images just because something newer and better has been released. With that in mind, perhaps the 5DIV will have enough of an improved feature set