As with many social apps, with Twitter your updates can be public or private. And anything that is public can be used for any number of things... Like, for example, this Web site TwittEarth.
(It might take a few seconds to load, but it's cool enough for the wait.)
What are you looking at? It's a Mashup (something that mashes two or more data sources, applications, etc. together to create a new application.) that puts real-time Twitter posts on the globe with monstrously cute icons. It is strangely hypnotizing and voyeuristic. And strangely comforting, in its own way, that normal people are doing normal things around the globe. For me, it makes me want to join in and be part of the global Twitter community. But...
In the back of my head, there is that privacy issue again. Why would I want to broadcast my every move to anybody and everybody? Okay, so maybe not my every move. I'm still trying to find that fine line of an public online life, with a relatively anonymous one and yet still not have a phone book's worth of user names and log ins. Sometimes it's enough to make my mind hurt.
This post from The Shifted Librarian explores two scenarios about personal privacy online. One is that sometimes it may be out of your control. The other is that maybe privacy doesn't really exist anyway, so why not put yourself out there? The Shifted Librarian also lists her choices for online user name registry for children and adults- as a means of protecting your name in the future.