If you were to ask any invididual what amount of lies, misinformation, and generally malicious/false statements someone has to make before they lose all credibility, I'm fairly certain whatever mean number you were to get from such a survey, Rush Limbaugh would be well beyond it.
Certainly, his having recently stated that President Barack Obama has not proved he is a US citizen casts a tiny bit of doubt on his credibility. But even setting that aside, it's not exactly a secret that Limbaugh is a partisan hack, a propagandist for the Republican party.
Which is no doubt why one of the "liberal" voices at Fox News, Greta van Susteren, featured on her program last night part one of a two part "interview" with Limbaugh. Susteren played the role of interviewer the way the "interviewers" in infomercials do: she set up Limbaugh and then listened attentively while he bashed Obama and opined on health care for long stretches of time.
If Susteren actually was a liberal voice at Fox rather than just another Stepford employee mindlessly selling Republican propaganda then she might bring on an actual guest with some kind of expertise on the subject of health care reform who would say something other than the Frank Luntz inspired Republican line about it. Someone like, say, Wendell Potter.