As rumors predict the launch of the new version of a product "soon", do you think it is interesting to wait for a few months before buying a new instance of the by-then "old" version of the product for its future hypothetical low price?
I would say no based on past experience. I was in the same position with the mkI mkII and what I observed was the price of used 5D mkIs didn't drop when the mkII was announced, nor did they drop when the camera was released and actually started shipping! It wasn't until a while after.
There are many reasons for this, one being demand. There were many people who wanted better AF, speed and weather sealing from the 5D successor. What they got was mote megapixels and video. So many didn't upgrade because of this (or took longer to be convinced).
Another reason is that people don't buy new cameras and sell their old ones right away, they wait for the price of the new cameras to drop a bit first.
Just realised you're talking about the new price not used. The same principles apply, price drops lag the release by some time. The price of mkIIs has been dropping since the release and are now way below the price point of new models. So even when the new one is released there will still be demand for the old one (because it is cheaper) hence the price of the old model is slow to fall.
That is a constant question in photography (and every other gear-infested hobby out there), and of course, it's not easily answered.
I'd buy a 5D mk II today, it seems to be a great camera which does everything and a lot more one could need. If you were a sports photographer I'd go with the 7D (higher FPS & the crop factor + sealing), but otherwise the 5D mk II is a solid choice. The ISO is crazy, you get out too many megapixels already etc. It will continue to be a great camera for a lot of years.
(and if the next 5D has even better ISO, think back to the film days and how ISO was then, compare it to your 5D and be
^_^ about it)
There is one simple reason why you should buy whatever camera is around vs one that is unreleased: you can use the one that's available right now, and you can sell it and upgrade later.
That said, there's no reason for Canon to release a 5D mkIII right now. It has the edge over Nikon in both megapixels and the ability to do 1080p video on a full frame sensor. It took 4 years for them to release the 5D mkII - soon after Nikon released a 5D competitor. There's no reason for them to go charging nilly willy into markets they already own.
I'm in the same position and have decided to wait. It's a pretty stable rumor that the mk III will be out end of year. I'm an oddball in that I really want more megapixels because I print to digital negatives for platinum paper prints. More megapixels actually are not wasted on me as I print at 360 lpm. So say the mk III ends up at the rumored 28 megapixels. That means I can make a 12"x20" print at 360 lpi. That alone is worth it to me to wait.
If I could afford it, I'd also look at the next-gen 1d/1ds that too which will likely have 35 megapixels (roughly a 14x24 inch print with that!) but that would also require me stepping up to a larger printer. Sometimes one wishes to be made of money.
If you're asking me, totally i will choose to wait for the Mark III version while we trust it is near to us that why you're here to ask this question. as you can see, how long period 5D Mark II is standing on the market and i trust there is something changing and improve. we're not getting at at very cheap price and able to throw it away while new version released as what happen to iphone.
anyway, for photograph. it is not something rely on equipment too.., it is just your tools but what important is you skill. not meaning that 1D should beat 5D down, theory is around there =D