I also wonder if they will be able to link wallets to vendors by tracking transactions to exchanges and getting subpoenas.  I.e. subpoena exchanges to get their deposit addresses, get all the vendor deposit addresses they can from their infiltration of SR2 (and possibly the SR2 tumbler), and try and connect the two.

People who are careful and mix multiple times may be ok, but there will always be a few who were sloppy.  Even if they don't capture the biggest and most sophisticated this way, I see a lot of PR value to government taking credit for "cracking" bitcoin's pseudonymity.

In this regard, how secure is Darkcoin?  It seems like the masternode operator can link transactions that they mix - although I'm not totally clear on this.  And there is no reason the gov't can't operate a lot of masternodes.

The response from Darkcoin seems to be that you can mix several times, and as long as one of the masternodes is not compromised, you are ok.  But it seems like the flip side is true as well.  If there are multiple compromised masternodes, then over time, they will capture the full transaction.