Wifi is such a cheap feature to implement
I think you are gravely underestimating the cost of adding WiFi. It isn't simply some chip you just buy and stuff into the camera. It has to be integrated into the camera!
JRista, I appreciate what you're saying, but I think you may be overstating the complexity. The reason it needs so much advance planning is likely due to the internal space constraints of SLRs. We know it's not "hard" because P&S and lower-end SLRs have it. The magnesium body, even on a 1DX is not 100% coverage: you could sneak a wire through that and put the antenna outside the magnesium, e.g. under the rubber grip material or on the prism bump. Hey, why not shoot for the moon and integrate WiFi, GPS and radio flash control in all future xD and xxD models! Of course it must be integrated, and it's probably more than $5 total cost per unit, but I'd have a hard time believing it's more than $25 per unit. Seriously, if Eye-Fi can put WiFi in an SD card, it shouldn't be that difficult. Any future SLR, even the next 1-series, really ought to have WiFi and GPS, even though they're not features I crave.
In terms of per-unit manufacturing cost, sure, I wouldn't expect it to be much more than that either. My point is you don't just drop in a wifi chip and be done with it. You have to design the camera with all these various factors in mind from the getgo, and for each additional feature you add, like WiFi, and GPS, etc. you increase the overall complexity of the product as a whole. The up-front R&D cost increases, the prototyping/testing phase increases, and the extra money spent on R&D has to be recouped somehow. So, maybe you do only have $25 in additional manufacturing costs...but the extra costs in R&D ultimately weigh on the final product price. Everyone complained about the $3400 list price of the 5D III when it first hit...but it really isn't surprising. A hell of a lot of thought and engineering went into making it (more so than the D800...Nikon farmed out the sensor to a third party, and based on the rash of complaints about ergos/button layout apparently really didn't put as much thought or money into layout as Canon did), and the price reflected that.
All I am saying is, the design and construction of complex devices like the 5D III tend to be much more complex than people inevitably reduce them to. Maybe it is just a natural trait of humanity, to greatly simplify things in their minds. Regardless, there is more to including WiFi in a new camera design like the 5D III than simply slapping a chip on a board somewhere and threading an antenna through a hole somewhere. It isn't just $5, it isn't just $25...it's part of a much more complex process involving years and dozens if not hundreds of people working thousands of man hours to research, design, and build the whole entire camera...and every single feature adds another order of complexity.