CarolC- Rush makes fun of linguistic idiosyncacies whatever someone's color or bent. Have you ever heard him imitate Bill Buckley? It's hilarious- and not at all flattering.
I make fun of someone saying, "Ax" instead of "ask" just as I make fun of our President saying "nucular" instead of "nuclear". I also make fun of people saying, "So don't I" instead of "so do I" in the area which I live in (and it happens to be a predominantly caucasion area.) Does that make me a racist?
My point? I don't think because Rush does a pretty good impression of Al Sharpton that he is a de facto racist.