It has probably been in development for ages. I'm sure they have already decided on which sensor they're going to use even before the D300 was released, let alone the D700.
But like ChrisA said, maybe Canon had to go back and look at the 5D MkII's specs again. Everyone says "a 5D upgrade is imminent", but we said this 12 months ago, because it really should have been released in January 2008, not a few months from now. I don't think Canon ever intended to leave the 5D without an update for so long.
It has been interesting watching the rumors on this one. I've been watching them since Nov 2007, when my Underwater camera (35mm film) flooded, which is prompting me to finally move to digital (BTW, that Tokina 11-16mm looks very tempting; too bad its not for full frame).
Insofar as "Concrete Rumors" on the 5D's replacement, this
URL at canonrumors.com has some claimed specifications, and also claims that the 5D isn't necessarily be 'upgraded' (aka "5D Mk2), but instead, potentially replaced with 2 cameras....although it should be noted that names are still TBD, so one of them very well could use the "5D Mk2" name.
In brief summary, it says $2K for a 14MP body ("3D") and $3K for 20MP body ("6D"), both Full Frame.
.....however...!
His latest rumor news (2 July ... yesterday) says:
"...about the new CMOS in the EOS-3 D. My source is claiming that Canon's new CMOS will once again bring Canon to the forefront in high ISO performance. I was told again sensor will be in the 18mp - 21mp range (I still think 16mp)."
Picking through the tidbits a bit further, these two statements also suggest interesting happenings:
(30 June):
When I put up the initial rumour about the 3D and 6D, some people commented that there's no way the megapixel count would reach what the 1DS is currently at.
This week we'll be seeing the Nikon D3 sold as a Nikon D700. So if Nikon can release just about the same camera for less money, why couldn't Canon?
And
(2 July):
The last words in the email were "the new Canon's will make every Canon user very happy."
Overall, I think that there's a lot of credence to what Abstract and ChrisA were alluding to above: Canon may very well be in the process of upping the "5D replacement" in order to compete better against Nikon, and it appears to me that part of their strategy has been to "hold off" on this upgrade cycle to see what Nikon puts out.
And since Nikon has really upped the ante, the prospects appear good for a very healthy kick upwards for Canon gear now too...I won't claim that it has been like our looooong wait back in 2001-02-2003 for the G5 CPU, but I won't say that it is unlike it, either.
-hh