The ideal solution is definitely to split ratings, both for upvotes and downvotes. Slashdot did this, and quite a few forum scripts have plugins for it, where you can rate posts based on things like whether you agree/disagree with them, if they're funny or informative, if they're spam/low quality etc.

Having it work that way would significantly discourage using upvotes and downvotes as agree/disagree buttons, and make it much clearer what the community thinks of any particular post or piece of content.