Laurel Papworth wrote an excellent post a few days ago about reputation management in social networks. I highly recommend you read it.
I'd actually go a step further and say that most of what she said is also true for your reputation on the Internet in general -- though that's a little more scattered and is therefore a little harder to gauge. Still, if you've been online for even a short amount of time and have participated in almost anything, you will have left a trace.
People can (and do) search the web for your name, e-mail address, phone number, alias/handle/nickname, and so on. And the deeper they dig, the more they usually find. For example, you can search for me using the following queries and can learn a little more about me each time:
Cool, isn't it? Or scary...depending on how you look at it. And the best (or worst) part of all this is that, once your information is up there, it's pretty difficult (read: darned near impossible) to get down.
To quote an already oft-quoted quote, Sun Microsystem's CEO Scott McNealy way back in 1999 said: "You already have zero privacy (on the Internet). Get over it." Yup. Get over it and, since it follows logically: manage your online reputation really well. Particularly since your online and offline reputations are increasingly crossing over -- that too in both directions.