I recently signed up for an account on myspace. Not to be a crumudgeon, but in browsing the site, I cannot help but notice that individuals post pictures and comments that they would never want known in the workplace. I then read a series of excellent articles by George Lenard (at George's Employment Blog) about how employers might use such content to screen out or terminate employees.
Would an employer be wrong to use such information in making hiring and firing decisions? Probably not, as long as it acted consistently without regard to an employee's protected class.
You are what you write.