It seems that some people are running into online image problems stemming from personal sites they didn't even create. Sarah White has the report of back-stabbing job competition among college students (via Ragan Jones):
There are cases of college students creating explicit or unflattering (to say it nicely) websites and user accounts pretending to be people they view as competition for jobs. Think about it - they aren't dumb. If they know someone will be looking to compare - who would you hire the one with a friendly site or the one very explicit. The sad part is - the other student may not even know the site exists, yet their reputations are tarnished.
How to protect yourself? First, look yourself up. Do the obvious, quick-and-easy background check that potential employers are likely to do. Look for yourself in social networking sites, too, especially if you're young enough for people to expect to find you there.
If you find something inaccurate—if you're the target of someone's unfair competition—you may be able to get the site removed (good luck). If that doesn't go anywhere, well... the legal issues promise to get interesting.
There's a lot more to your online reputation than a MySpace profile. If there's adverse information about you out there that won't go away—even if it's false—you can use other services to counterbalance the bad stuff. Start a blog; post a LinkedIn profile; post some pictures on Flickr (not the ones that create more trouble for your job search!); create a competing MySpace (or Facebook, or whatever) profile. You might even write about the false profile (but don't air your suspicions about who may have done it online). The goal is to bury the bad profile with more favorable search results, but you'll still want to get rid of the bad stuff if you can.
This sounds like something that's happened once or twice, but even assuming it's unlikely to happen to you, you should know what people find when they look you up. You knew that by now, didn't you?