Eliezer's post Conjunction Controversy (Or, How They Nail It Down) also covers some of the research showing that the conjunction fallacy is not just due to misunderstandings.
It could be in part due to training.
Reyna & Brainerd's Fuzzy-trace Theory research suggested that the conjunction fallacy gets more common with age.
And given that some of my guess-the-teacher's-password heuristics more or less directly invoke conjunction fallacy (Given A, B, C, and D, where D is A+B, D is more likely correct than either A or B), I'm inclined to suspect that education might be strongly reinforcing the bias.
The chapter on judgment under uncertainty in the (excellent) new Oxford Handbook of Cognitive Psychology has a handy little section on recent critiques of the "heuristics and biases" tradition. It also discusses problems with the somewhat-competing "fast and frugal heuristics" school of thought, but for now let me just quote the section on heuristics and biases (pp. 608-609):
My thanks to MIRI intern Stephen Barnes for transcribing this text.