(Actually I mostly forgot that he even had a television program - me and the vast majority of American viewers, according to his ratings.)
It seems that the major distinction is that while Imus made an inappropriate remark in general, Letterman made a remark which is intended to personally hurt a politically conservative figure. And since he works for a network which is not friendly to conservatives, in a field which has many anti-conservative leaders, in a city which is certainly left-of-center, he can get away with it. Yes, Virginia, there is a double-standard in our culture: there are those who adhere to ethics and those who do not. And every time that someone who advocates a certain moral standard can be attacked, it is appropriate to do so.
But let's not assume that the personal difficulties found within the lives of those who advocate moral standards diminishes the standard itself. (Since the Palins identify themselves as Christians, we can easily work our discussion from a Christian perspective.) Those who advocate moral ethics usually do so on the basis that God has written his law within creation and bestowed his Spirit to guide us. Along with this worldview is the assertion that, while his creation was made good and his law remains perfect, those to whom creation and law were entrusted have become distorted. Thus, failure to live up to the standards of divine morality does not tarnish the law but rather affirms the narrative which surrounds the giving of the law in the first place.
Although it is common for those who wish to do whatever they want - so long as power, fame and money are coming in - to take opportunity for personally attacking the lives and families of those with whom they disagree, it does not in any way corrode the moral code itself. When we realize this we are then able to see the mean-spiritedness of those who choose to cackle at sexual references regarding young women less than three times their own age. The only achievement here is that people are indeed speaking about Letterman's show again, something which hasn't happened in a good many years.
Notice that you will not hear these comments about Malia and Sasha Obama (most people don't even know their names because the Federally-controlled media has obeyed the directive not to speak of them). Double-standards apply, but they are far greater than the Republican-Democrat or Conservative-Liberal lines . . . it is a matter of ethics and worldviews which either are or are not founded upon the Divine.