How?  BTC transactions are irreversible.

I doubt that this would happen.  The Bitcoins would not have the same address.  They are not the same Bitcoins.

Maybe Bitcoins obtained straight from a stolen source are one thing, but Bitcoins that have transferred MANY times with differing amounts over a year?  What basis would that be?  They do not do that with cash or currency. 

There is no anonymity in Bitcoin against powerful entities:

https://bt.irlbtc.com/view/439357.msg5355485#msg5355485

Incorrect. You had to transmit your public key to the sender who sent you BTC.

Edit: actually the public key is hashed and not public until you spend. But unless it was sent encrypted to the sender, this could be intercepted. And frankly most of the time you are not obtaining coins from such a covert sender. When you buy on an exchange, the authorities can get this information.

Also once you spend your coins then your public key is revealed. If you can't spend your coins without revealing your identity, then what is your point?

There is hundreds of years of case law that already settled this question.

In most jurisdictions (Switzerland being an exception) if you purchase stolen unique goods, such as a painting or a sculpture, even if you didn't know that it was stollen and you acted in good faith, the legal title for these goods remains with the original owner. 

This does not apply, however, to non-unique or fungible items like dollar bills or bitcoins.  If your restaurant receives payment in bills that were stolen from a bank robbery, then these bills remain your property provided you received them in good faith (i.e., you weren't in on some sort of money laundering operation).  This applies even if the serial numbers of the stolen bills are known.  If you think about it, a cash economy couldn't function any other way!   

Another note is that, even after a few transactions, you can't really refer to "this bitcoin" or "that bitcoin".   Let's say Alice unknowingly receives a stollen 1 BTC coin.  She then sends that 1 BTC to Bob using the Blockchain.info SharedCoin feature.  Shared coin merges, for example, ten 1 BTC outputs (all from different people) into 1 transaction, and then send those bitcoins out to, say, 10 different addresses.  There is no longer a blockchain record that shows that Alice's address sent 1 BTC to Bob's address.  Instead, Bob's bitcoin has 10% taint with the bitcoin that Alice sent.  Now, if Bob sends 5 mBTC from that coin to pay for a coffee, and then the merchant moves all customer payments for the week to a new address (and thus likely into a single, say, 5 BTC coin) this new coin shares 0.01% taint with the original stolen coin that Alice unknowingly received from MtGox.   

Pretty soon, the taint has diffused throughout the economy!

As an experiment: go to blockchain.info and look-up one of your bitcoin addresses and select the "taint analysis" option. I bet there is taint linking it to hundreds or thousands of other addresses.  There's a good chance that your address has 0.0023% taint of a coin used for something illegal.  Just like a few of the twenty-dollar bills that you'll use this year had at some point in the past been used to pay for marijuana.   

In conclusion:
- If you receive stollen coins in good faith, they become your legal property. 
- After several transactions, a 100% stollen coins get fuzzed-out across numerous coins, each sharing a small amount of taint with the original. 

AnonyMint, in one sentence, is this your position?

Since taint tends to diffuse, spreading out to more and more coins over time, and since at least some coins were involved in a crime at some point in the past, AnonyMint claims that most bitcoins in circulation would become subject to confiscation.

WOW... the lack of common sense is truly a sight to behold!

Quite simply, this WOULDN'T happen with cash - so therefore wouldn't happen with Bitcoin

....I don't see the confusion

Cash = legal tender.

Gold != legal tender

Gold can be confiscated if you buy it and it was stolen property.

Can't you read my prior post, or are you fucking blind?

Legal tender is given this protection, because the government provides consumer protection against theft, e.g. FDIC insurance, tracking down serial numbers on bills, chargebacks on credit cards, etc.. It is their racket, and so they give it is special status.

How would you propose they return coins to the victim of a theft if his coins are now distributed as taint across 24,000 other coins?  Say Sally has 1 BTC that has 0.002% taint of a coin that was stolen from Sam.  Would they correlate Sally's human identity with her 1 BTC wallet, find her somehow, and then ask for 0.002% x 1 BTC = 2000 Satoshi's back?  And then they would find the other 23,999 coins and do the same?  And what if Sally was also the victim of a theft at some prior point in time, but the Coin Rescuers hadn't found her coins yet.  She might be upset that you're taking some of her coins, when she was a victim herself.  Do you think you could add an appeal process so that victims waiting for their settlements could defer having a portion of their coins removed?

What would you propose they do if later it's found that this 2000 Satoshi restitution paid to Sam was actually tainted by 10% with a crime from many years ago that was just recently solved?  Would they than go to the Sam, to whom they just return 2000 Satoshis, and confiscate 200 Satoshi's to give to Joanne?

Are you imagining that they automate this in some way?  Are do you imagine that a significant portion of the population is involved with splitting a few pennies off of coins to provide restitution to victims?  You could really cut down on unemployment if you did this all manually.  And since you have people tracking other people down for 2000 Satoshi's  = 1 penny (due to diffused taint), and this process probably costs more than 1 penny, how would you propose we pay for this?  Maybe the Coin Rescuers could also control mining and most of the nodes, and then they can just issue as many bitcoins are necessary to pay their workers to rescue all the pieces of the stolen coins distributed as pennies across the economy.  So you've achieved near 100% employment and everybody can get their stolen coins back.  

And who are they by the way?  And do you think they are watching you through your webcam right now?

Wait, I'll admit I'm late to this game, but it seems to me that a block chain can be corrected (superseded) after it is created.  Is there a point where that cannot happen?

It seems to me that a theft, if proven, could cause the chain to be reinstated without the theft.