Buzzbees comments are the best I've heard so far.
No single chemical or pesticides have been found to be the problem. In fact, some of the ones targeted, and those banned in France and other countries, have been found at ANY level, in only about 30% of CCD samples. Some of the other chemicals, and the ones beekeepers have placed in the hives themselves, have been found at much higher percentages of the hives. Fluvalinate, being the most toxic.
It seems that particular fungicides, tested and shown to be safe for bees unto themselves, has drastic kill rates, when mixed with other chemicals. These previously safe fungicides are being found to be the the "x-factor" and chasing down and eliminating one chemical as they are trying to do, may be in vane, since another chemical may just fill in for the job. So this could be a compounding problem that results in many chemicals being the "last straw" allowing this to take place.
It seems that research is about where many beekeepers thoughts were at least two or three years ago. That it is the combination of chemicals, probably with suppressed immune systems, poor nutrition, that allow other diseases and a complete crash of the bees immune system and resulting CCD.