I'm running on of memory here, but as Angellous is participating here I can get away with such laziness counting on his corrections (totally unfair of me, of course).
If memory serves the first time Mary Magdalene was explicitly identified as a prostitute was by a fairly early pope (Gregory?). However, I seem to recall some arguments that there are indications that in the gospels which (if historical) support to some extent this interpretation. One is simply how she was identified (not by relation to a male but to a place- Magdala). This may indicate not just the fact that she was unattached but had some independent wealth (something which allowed her to operate independently in a misogynistic world where women typically couldn't, and even wealthy women were connected to male supervisors).
Additionally, while the "demons" Jesus drives out are often referred to as possessing (or possessed by) pneuma akathartos, incuding the other women among whom Mary Magdalene is singled out in Luke, she herself is said to have had daimonia or demons. There seems to be a distinction. For one thing, daimonion need not have the adjective akathartos or unclean as pneuma does. Second, by NT times (and in christian literature) it is almost always associated with evil or wickedness. It's true that it does describe mental or physical illness as well, but as these were often seen as the product of immorality, one can't equate sickness and daimonion. Also, there although it is likely that many thought to be possessed were mentally ill, they could also simply be thought immoral (or physically ill). In other words, a sick person might be under the control of an unclean spirit or a demon, but so might a person who performs wicked deeds or acts immorally.
