Linda Gorman has done a nice job debunking the commonly promulgated myth that "that half of all bankruptcies are caused by medical debt".
This dovetails nicely with testimony given by Todd Zywicki, Professor of Law at George Mason University School of Law, to the US Congress on July 17, 2007 House of Committee on the Judiciary, Subcommittee on Commercial and Administrative Law. His testimony is available here as an MS Word document.
He also concludes that, "There is No Evidence That There Has Been An Increase in the Frequency or Severity of Job Loss or Income Interruption as a Result of Health Problems".