Sure, if we resist power in certain contexts, that supports the claim that we have the capacity to resist power. However, we need to talk about more than "human nature" here: if we want to figure out why that behavior emerges, we'll need to analyze the surrounding context too.
This is distracting us from Foucault's point, though. He's not saying "There is no such thing as human nature"; he's saying "Let's study the ways in which people construct knowledge about human nature and see what roles those forms of knowledge play in sustaining power relations". Note that Foucault's point is completely orthogonal to any claims about "real" human nature. --It's like sociology of religion: we can study the role of Catholicism in a society, but whatever we find out is irrevelant to the question, "Are Catholic beliefs true?"
So studying human nature and studying the ideology of human nature are both important endeavors. The latter fascinates me, but I won't begrudge people who want to tackle the former.
Miles