I'm going to say some things that you might not like - I definitely don't like this idea but...
As time passes by - more and more people get interested in Bitcoins, more and more thefts will happen. If we start marking Bitcoins as 'dirty' - some time later we will have almost 99% of the coins being marked as bad.

What should an average Bitcoin user do if 99% if his income is 'dirty'? Is he actually responsible for some event that had happened long before he even got to know what Bitcoin is?

Tracing stolen coins and returning them back to the owner is good at first or second step of transacting those coins. And only if you are 100% sure that previous owner lost them and current owner have stolen them.

As it have been said previously - money is money. Community should try to block theives but we shouldn't go absolutely crazy about it. And I think that community should accept this concept as default. This will be hard to do - exactly as it is hard for someone to accept Bitcoin as the completely new paradigm. But if we don't do this now - we will arrive to that point anyway: Bitcoin stands for new opportunities but it also brings new problems. And people will have to learn to live with them.