Recently, I came across an article by William Howard of University of Alaska-Fairbanks in the Journal of Chemical Education. There is an HTML version here.
Howard's basic point is summed up in his last paragraph - "knowing all of the products from the potassium-argon reaction is necessary in order to know the age of the mineral [being dated]." Howard says that nobody has ever studied the 40K- 40Ar reaction in minerals enough to know all of the products. For this reason, he says it's impossible for us to be sure that radiometric dating is accurate.
To illustrate, Howard gives the following analogy:
"A stocker at a local grocery store is given the task of stocking store shelves with cans of corn. The stocker removes 10 cans of corn from each carton, places the can on the shelf, and discards the empty carton in a nearby waste bin. The stocker works for awhile and then breaks for lunch. A store manager inspects the work after the stocker has left and wishes to know how long the stocker had worked...[T]he manager finds that there are 100 cans stacked on the shelf, but only 8 empty cartons in the waste bin. Can she draw a reasonable conclusion concerning how long the stocker had worked?...Now imagine another case, in which the manager counts the 100 cans of corn on the shelf, but she does not bother to count how many empty cartains are in the waste bin. She concludes the stocker must have worked for 10 minutes. Is the manager's conclusion reasonable?"
"Just as the manager must count cans of corn and discarded cartons in order to judge how long the stocker had been working, so must the chemist identify and quantify all products from a reaction in order to deter-mine how long the reaction has been going."
Howard says that since we do not know know the details of the 40K - 40Ar reaction, including all products, it's impossible to know that we receive an accurate radiometric dating, because it's impossible to know if there are any other factors (like, perhaps, some other non-radiogenic sources of 40Ar in the mineral) that cloud our measurements. He also states that this applies to other methods of radiometric dating, so even independent verification by other radiometric dating methods does not validate the accuracy of any of them.