Ms. Fluke is likely a “limited-purpose public figure,” since she put herself in the middle of a public debate over a mater of public controversy. This might not fully protect Rush (as Ms. Fluke’s lawyers would argue that his comments about her personal sexual behavior went far beyond her testimony, which concerned other womens’ use of contraceptives for medical purposes other than preventing pregnancy), but it would probably be difficult for Fluke to win a defamation case here.
Of course, if she sued, Rush may be forced to settle - the MSM would spin any defense he presented as “defending his right to call a weak, defenseless student a slut,” and it may ultimately be more costly to Rush (as well as his advertisers, etc.) to fight the lawsuit than it would be to just settle quietly.
And, in a few years, she will be as venerated as Cindy Sheehan is today.