They get tested, but that's about it, and to be truthful it really isn't much of a help. STDs run rampent in the porn industry.
You might remember Sharon Mitchell from her porn movies many years ago. She went on to get her phd and started AIM a health care foundation for adult performers. This is a quote from an interview she did on court tv:
Adult Industry Medical Health Care Foundation
http://www.aim-med.org/
If you are considering doing porn, I highly suggest visiting the site for info. Aside from health info, she has a lot of advice to offer about the porn industry.