One of the biggest issues is that once you make a transfer you combine coins from multiple addresses and as a result those can be identified as one wallet. I think casascius proposal addresses this only to some extent. If after mixing coins I again have to combine I have gained nothing.
How about we do it different:
Whenever I want to make a transaction, my client sends this out as a "transaction-indent", other clients that are also about to do a transaction combine their "indent" with mine (adding inputs and outputs) and after a few seconds, we all sign this combined indent to form a transaction.
This would make it impossible to identify a single wallet, because inputs from multiple wallets would end up in a single transaction. And secondary, on one would know which input was the initiator for which output.
Your comments?