Hello... I am the administrator of the site at
http://www.crl.com/~mjr/thrower.html This site is dedicated to all sorts of thrown objects for sport, self defense, etc. It also provides a link to the thrower mailing list about the same subjects. Note that the thrower list has MOVED to a new host as of today (6/25), so If you want to join, or tried to join any time in the last couple of days, please revisit the site and do so again...
Now onto the topic. As others have stated, you can throw just about anything. However, a good "throwing knife" will be fairly heavy, a rule of thumb being 1 oz. for every inch of length. Knives that are too light (say less than .75oz/in tend to float, especially at longer distances and not rotate consistently.
Being "center balanced" is not that important, though center balanced knives do have interesting characteristics. A knife's balance (center, or very handle or blade heavy) will influence its flight, but once you know what the flight characteristics of a given blade are, like the man said, you can throw anything...
One last thing though... Knives made to be thrown tend to be much softer than "real knives". Experiments have determined that RC 44-48 is about right for a thrower. Lower than that and the torque of impact will BEND the knife either at the tip, or put a bow in it. Much harder, and it will be too brittle to stand up to many throws.
I'll end here to keep it short, my very first post on bladeforums! Take care and feel free to ask what ever you like, visit the web site, joint the mailing list, etc. I've been throwing knives for about 6 years now but my real claim to fame is having the first web site and mailing list (still the only mailing list as far as I know) on the subject...
Take care.