my take on this ---
the d600 with those specs and build is no real threat to canon at $2200. If anything, its a bad move on nikons part because the price gap between it and the d800 is too low.
These moves will hurt nikon in the long run I think. The d600 is priced for profit and the d800 is priced at a loss and if you notice the differences in features the d800 is well worth spending the extra $800 on. If the canon 6d ends up running between $1800-2300 and keeps the price of the mk3 around the same rate (maybe drop it to $3300) then nikon better watch out! Yes, d800 's will sell like hot cakes, and the d600 will get passed over by most because what do you get for the extra $800? Weather sealing, 36 mp's, more ISO options, etc etc etc.
i guess it all depends on the accounting --- is the d800 priced so low because nikon is taking a loss, or, is it due to the deal with sony (IE, less R&D for nikon). these are things we probably will never find out. But, what we will find out soon enough is what the actual specs and price of the 6d will be. If it is priced at $2299, there won't be the big i'm jumping ship situation (the d600 is a nice looking camera, but it's no game changer) ---and I highly doubt the 6d will be a game changer either --- both are more targeted at the consumer crowd, not the pro crowd (with the exception of some pros who may snag either as backup bodies).
I guess we'll see!