Whether it is directed towards a Republican or Democratic President, it is the ultimate insult. On a PC basis, it is inexcusable, and perhaps slightly worse when directed towards Obama due to the racial/genocidal implications. As much as I disdain Limbaugh and find it regrettable, I don't think censorship should be applied. Hyperbole is a key element of political criticism, and while I would hope those who use it would employ it sparingly for risk of weakening its effect, it should remain on the plate as legitimate tool.
A few months ago I may have opined differently, but after living in Mexico and seeing the dominate control over the media, especially during the recent elections, I have to come down on the side of free expression. Let's hope there is responsibility that accompanies it, or such hyperbole becomes cliche or transforms into vitriol and rancor. Just my .02 cents.