We will see what Letterman's rating do over the next few weeks or months, if they decline, he may be fired, if they increase or are stable he will not be fired.
Most importantly, both CBS and Letterman's advertisers can review the ratings book's cross-tabs for the show and determine if there is an overlap between social conservatives offended by Letterman's comments and and social conservatives who regularly watch Letterman. If there is no overlap, Letterman will not be fired. My guess is that there is little overlap.
That wouldn't surprise me at all.