There's an obvious difference between rappers and Imus. Rappers may well use the same words he used generically in songs (I don't know, I don't listen to rap) but Imus directed the words against specific young women, who were not public figures, and who had done nothing to attract his emnity. I do see a difference between referring generically to "hos" and whatnot and singling out specific individuals.
Imus' remarks might have been excusable against a public figure -- after all, if you're well known, you can just call a press conference and attack Don Imus. But 18 year old college girls can't easily do that. That's bullying, and that's why he should be fired.