Here's something kind of scary: The government is going to start collecting DNA samples from everybody they arrest. Let me repeat, you do not have to even be convicted of a crime and your DNA will end up in some database. You know how easy it is to get arrested? I think it goes without saying, but clearly I am against this out of privacy concerns.
Remember, small steps lead to big changes. The government will then start requiring DNA to get passports. Then to get drivers licenses. Then when you're born.
Think about the future technology. You shed dead skin and hair follicles all day long. Pretty soon there could be scanners that detect not only where you are, but where you've been! Sure it is probably a long ways away, but before you know it, the future is here and all those little steps that slowly took away your privacy (and/or freedoms) have now culminated into one giant step from the beginning.
Please reference my post about RFID Passports if you think that one with nothing to hide has nothing to fear.