A digital currency system like Lucre (OpenTransaction) creates a coin by a mint blind signing the output of a hash function, which the payer then unblinds and pairs with the input to the has function.
In this way, the coins themselves cannot be used to identify the payer; however, the payee and the mint are free to correlate their information about the payer's identity through side channel information, such as IP address and rebuild the payer's identity. There are afaik two orthogonal partial solutions to this problem :
The payer may reduce side channel information, say by using Tor, not giving one party a real name, etc. This potentially offers good security, but imposes a great burden upon the payer.
The protocol and payer may instead insist upon an intermediary between the payee and the mint, i.e. the payer blinds the coin using his intermediary's daily public key, the payee blinds the coin using the mint's key, and the intermediary unblinds the coin before delivering it to the mint. This prevents the mint form ever reestablishing that the payer held that particular coin.
I could imagine a few uses for #2 but I haven't seen much discussion about it. Is it interesting?