I don't defend what Letterman said, i also don't watch him, because i consider his "humor" to be based in nasty, invidious comments about individuals. I've felt that way for 20 years or more, so this episode doesn't surprise me at all.
With that caveat, i'd say that Letterman is not likely to be fired because there are so few people who are genuinely pissed off about this, and for whatever my taste in the matter may be, he gets high ratings. Imus made a racist remark, which offended far more people than Letterman's remark has, and one of the women referred to brought suit against him and against NBC. Imus may have enjoyed high ratings with his radio show, but nothing in comparison to what Letterman is worth to advertisers with his ratings. Imus also had a prior history.
In 1984, he referred to Howard Stern as a slut, and a Jew bastard who should be castrated. He referred to a black sports reported at the New York Times
as a "quota hire." In 1993, he referred to a female reporter at the New York Times
as a "cleaning lady." One columnist from the New York Times
reported in 1998 that off camera in a "60 Minutes" interview, he referred to his radio sidekick as having been hired to tell "n*gger jokes." Robin Quivers has said that he called her "n*gger" to her face, and she and Howard Stern both claim that he screamed "n*gger, n*gger" at a black woman employed at WNBC. Imus has referred to Arabs as radheads too many times to count. His station had trouble employing female newsreaders because of his comments on their appearance and he referred to one newsreader as a "skank" on the air. In 1998 he had referred to a critic as a "little Jew boy" (with other grossly offensive descriptors) and in 2004 referred to the owners of the Simon and Schuster publishing house as "thieving Jews." In addition to using slighting terms and racist epithets for Jews, he has offered the same kinds of remarks about the Irish and the Italians.
There's just no comparison between Letterman and Imus, the latter of whom is a loose cannon with a big mouth which he has shot off offensively for years and years. Imus has been employed since the Rutgers flap, and has not learned to keep his mouth shut. He made a racist comment about Dallas Cowboys defensive back Adam Jones.
There is just no reasonable comparison to be made between these two. Once gain, i am not defending Letterman, whom i happen to despise as a comedian. But it is ridiculous to compare this incident with Imus' life long history of insulting and racist remarks.