A friend of mine was conducting a job search for a graphics manager. He had it narrowed down to six people he wanted to interview. He did a search of them on MySpace and noticed one of them had pictures of himself talking about his drinking and drug-infused partying lifestyle. Even though the was extremely qualified for the job, he didn't get a phone call to be interviewed.
Job seekers, not only should you control how you're portrayed on such sites, you should also do a web search for yourself to see what comes up. Your potential employer may likely be doing the same thing!