This is one of Bitcoins biggest questions right now, what is a tainted coin? It seems that it already are some tainted coins out there that for example Mt.Gox is not allowing to be traded with at their exchange, how does one know if they have some tainted coins in their wallet? And also who dictates if a coin is tainted or not?

This community should have a very clear answer to this questions, no one serious will invest in this Economy in the long run without a clarification and Bitcoin will just attract small players and hackers.

If someone is giving me fiat money, I know that I can spend that money as easliy as I got it. The same thing should be with Bitcoin, so how are we going to handle with this issue?