I would think that it was traced via bitcoin, not pgp.
Shrem is screwed, because all those emails are highly incriminating with him actively helping BTCKing avoid being flagged in the system for laundering checks. BTCKing was originally caught because he's a drug addict supporting his habit by exchanging coins and was completely sloppy and careless with everything.
- seize drives or papers with private keys
- extract printer memory if they printed a private key
- subpoena exchanges to hand over records to tie them to identities they have on file
- search if somebody put the address in a forum sig or on IRC/elsewhere and admitted it was their address
- check IP if they generated the address with an online wallet, brainwallet or some other discoverable method (not behind Tor)
Otherwise all you are looking at is a big chain with coins going from address to address and no way to prove somebody is the definitive owner of it.
There is almost no such thing as two completely anonymous people in a financial transaction. There's always an email address, user name, a third party, or postal address to tie them.
The only thing that I can think of is some type of charitable/political donation that is anonymous.