I like the idea. I could imagine a client with an "Automatically mix coins" advanced option in a menu, along with a quick dialog box about the anonymity benefits
+1 I love the idea OP, but it should probably not be a mandatory function of the Satoshi client. Better as an option built into many clients.
The reason it shouldn't be a mandatory part of the Satoshi client is that that client should strive to avoid feature-creep. There is value in simplicity, especially for the core client. But if this option became standard in the Advanced tab of all clients, I'd be a happy man
