What she said doesn't surprise me, but I am surprised by how many people seem to be surprised. It's not worth blogging about except that James Taranto has a larger point about why it's funny to the Left.
The answer, it seems clear, is that this is an example of shock humor: a genre that relies on the frisson of violating taboos. By our count, Sykes runs afoul of five taboos in her Limbaugh joke: She equates dissent with treason. She likens a domestic political opponent to a foreign enemy. She makes fun of the disabled (Limbaugh's past addiction to painkillers would entitle him to protection under the Americans With Disabilities Act). She makes light of a form of interrogation that some people consider torture. And she wishes somebody dead.
Except for the last one, these are all taboos that liberals promote and enforce with especial vigor. If a conservative violated any one of them, he would be on the inside track to be named "Worst Person in the World" by that NBC blowhard (as indeed Feherty was).
What makes Sykes's joke funny to a liberal, then, is the sense of danger that accompanies her risky themes, combined with the secure knowledge that since the joke is at the expense of a liberal hate figure, the usual rules do not apply. It's the same reason people on the left evince particular glee when they attack Clarence Thomas or Michael Steele in expressly racist terms, or when they use antigay innuendo against their political opponents (regardless of the latter's sexual orientation).
One of the great PC tricks of the Left is making certain words and thoughts by the Right taboo. They reserve the ability to go down those roads to attack members of the Right before locking the thoughts and words up again for the sake of decency.