I can't opine on the connections between his philosophy and fascism; the fascists appropriated environmental issues, but this is not to say that concern for the environment makes you a fascist. Equally, we must all remember that the influence of the class system is real and deep, and that even people we admire deeply and who had all kinds of truths to teach, Arendt, Wittgenstein, ....were elitists.
Derrida lost me in two ways: First, in making text a metaphor for everything (while ignoring the rich history and universe of meanings related to texts); second in his insistence on the primacy of the referential function of symbols when in fact what symbols primarily represent is not their correspondence to some thing-in-the-world but the troth between reader/writer or speaker/listener. It is not in symbols that we find certainty, but in our relationships to one another.
Now while I agree with Carrol that perfectly odious people can speak truth -- who better than Hitler could capture the counterrevolutionary spirit of the thirties: "Better an end with horror than horror without end!" -- it's also the case that my actual encounters with Derrida and Foucault (they both came to UCB to lecture) did not inspire anything but distrust.
Joanna