Interesting.
http://www.drudgereport.com/mattabt.htm
Personally, I think this is nothing more than a publicity stunt on the part of Air America because of their....well....ratings, or rather the lack thereof. I read an AP story not long ago that pointed out in NY the ratings on the station they took over, which previously had an "all caribbean" format, went down from a 1.4 to a 1.2, and finished 26th in the market. The same article pointed out that the same thing is taking place pretty much everywhere it's on the radio.
That aside, an understanding I didn't hear the skit itself, was it out of bounds? For decades it has been the law that direct threats to the President are not protected free speech, but I do not know how far it has been challenged in the court system.