There's a reason why cash-in-the-mail is well used ... privacy for the sender remains intact.
So if you are buying bitcoins with cash-in-the-mail, your transaction remains quite private.
But if you are cashing out your coins, cash-in-the-mail to you does not necessarily protect your privacy from the sender knowing who you are.
The challenge in trying to transfer a physical item without the sender knowing the recipient is something seen quite often in spy movies. The same techniques applies though for truly anonymous cash transfers, including the tactic of employing a dead drop:
- http://en.wikipedia.org/wiki/Dead_drop
There are secure communications methods (e.g., GibberBot + Orbot on an Android mobile) that can let two counterparties conduct a trade without ever needing to directly interact.
Certainly though, there could be restrictions against transferring cash in this manner in many jurisdictions -- particularly with larger amounts of cash.
Now if in instead you simply meant how can you trade without showing an ID, there are many methods.
Bitcoins Direct now takes cash deposit at several banks. BitMe.com accepts cash deposits at Chase. Individual traders on LocalBitcoins.com can be found.
And for cashing out, LocalBitcoins.com works for that as well.
Another option instead of simply converting to cash for everything, bitcoins can be spent when making purchases. If the places you would consider spending your coins at don't currently accept bitcoin, each reminder the merchant receives about bitcoin will help make the day where they do accept bitcoins come sooner.