sorry-- for further reading, check out his article at
http://www.blackwell-synergy.com/doi/abs/10.1111/1467-8721.00181

In it, he argues that part of the issue is that we still, in some ways, view the human mind/brain as the "best," rather than just the combination we've ended up with. "The overzealous efforts to dismantle arguments of human uniqueness have only served to show that most comparative psychologists working with apes have yet to set aside the antiquated evolutionary "ladder." Instead, they have only attempted to pull chimpanzees up to the ladder’s highest imaginary rung — or perhaps, to pull humans down to an equally imaginary rung at the height of the apes. A true comparative science of animal minds, however, will recognize the complex diversity of the animal kingdom, and will thus view Homo sapiens as one more species with a unique set of adaptive skills crying out to be identified and understood."

Daniel Povinelli, formerly at (I think?) the University of Louisiana, did a whole book's worth of experiments on things like this called "Folk Physics for Apes." In it, he argues that a lot of claims about primate Theory of Mind and general intelligence is the result of wishful thinking/observer bias. He then does a whole heap of experiments which seem to indicate that primates are not at all as smart as we think they are. he attributes this to the fact that researchers come to love the little guys, and think of them as "just like little people."
For instance, he did one series of experiments in which chimps were unable to understand that researchers with buckets over their heads could not see them.