OK... alot of clueless answers here.

Category doesn't matter. One article in multi categories, is still one article. Categories are for people to be able to find content within your site easier. Google still sees it as one post.
Real world example... any of the blogs at weblogs inc. check and see how many cats each article is in, then tell me google is penalizing for this
of course if you use cats in your permalinking then you may have a problem, as google would see it as different articles url wise.
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