Algorithm to determine how positive or negative a statement/text is

Question

I need an algorithm to determine if a sentence, paragraph or article is negative or positive in tone... or better yet, how negative or positive.

For instance:

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Answer 1

Maybe essay grading software could be used to estimate tone? WIRED article.
Possible reference. (I couldn't read it.)
This report compares writing skill to the Flesch-Kincaid Grade Level needed to read it!
Page 4 of e-rator says that they look at mispelling and such. (Maybe bad post are misspelled too!)
Slashdot article.

You could also use an email filter of some sort for negativity instead of spam-ness.