But: this all makes the bold assumption you can absolutely tie an address and specific Bitcoins with an individual or entity.
It won't happen, I can assure you. (Some hints: connecting through Tor; building OpenTransaction overlay upon bitcoin > exchanging blind tokens, etc)
You dont allways have to tie them to a person or entity, its enought to tie them to a crime to taint the adress they are at and maybe any adresses they go to.
One problem though is speed, it has to go quite quick? What happens when it goes to slow...Before an adress is tainted, the coins are allready moved into an exchange and ends up spread to other users.
Should those user be asked to send the coins back otherwise their adresses will be tainted? Thinking about it, its not reasonable...
So this tainting system should perhaps only be there to prevent crimes such as extortion and kidnapping from being based on Bitcoins, because in both cases you will be able to know in advance that a certain adress will contain tainted money.
Ofcourse a kidnapper could allways threaten that the adress should not be tainted or else.
But in these case I see governments cooperating with exchanges and warning them of the adress without tainting it.
Though thinking further would not be possible since he could send the coins to a new adress, exchange them at an exchange and claim that he bought the coins from someone unknown etc.
Thinking about the speed...
Thats where whitelisted adresses comes in, or known adresses.
Exchanges will have to only accept coins with zero day delay from whitelisted adresses.
Any unknown adress will have to be in quarantine waiting to be aknowledged for say 3 weeks depending on country, so that its not money from a serious crime.
Some poker networks have these kinds of quarantine when withdrawing funds.
This will also force real users to rather use and keep their coins at whitelisted adresses.
An adress will probably be "whitelisted" just by doing a verification at any of the exchanges.