the mark IV could be at lease 6months away, minimum. however i have noticed a $300 price drop in the mk III recently......hmmmmm
6 months? That seems highly optimistic given that we don't even expect the 7D to be announced for another 6 months.... 2 years seems like a more realistic timeframe... but I would bet on longer than that.
As digital cameras mature, the difference between updates gets smaller and smaller and for the higher end cameras you can expect update times to get longer.
I'de agree with Don, you could be waiting another 1 to 2 years for an update to the MK III, Cameras are throw away items pretty well, buy what you can't afford now, sell it, or give it away in 2 years and get the new one, don't fall in love with them as after 2 years your always looking over your shoulder at the newer model that just walked by, and be happy with NGAS, this is not an affliction, it's a well developed and nurtured condition.
If you really wanted to upgrade every 6 months, get a Nikon D600, then you could get a Nikon D610 within 6 months (actually right now), and if the optional oil chucking shutter isn'y fixed, you could get the Nikon D620 maybe by Xmas next year, that would be 3 new Cameras in 12 months, with or without the problematic shutter (your choice of course) but all NEW !! and all with no discount for the previous faults in design, Nikon could be onto something here.