Freakgirlspew teaches me a new word:
dysphemism (DIS-fuh-miz-em) noun. The substitution of a harsher, deprecating or offensive term in place of a relatively neutral term.
It's the opposite of euphemism. The example that this page gives is 'A guerrilla in neutral language might be called freedom-fighter by some while a terrorist by others.' The all-knowing wikipedia expands on this:
Oddly, some humorous expressions can be both euphemistic and dysphemistic depending on context: for example "spank the monkey" might be used as either a softer alternative to "masturbate", or as a more deliberately provocative one depending on the audience.
As I'm feeling thorough, here's a third definition with examples.