This subject kind of came up in the Newbies forum when one poster advertised wanting to buy bc for cash in Amsterdam and a respondent suggested using an exchange, doh.
The problem with the "exchanges" is that they are not actually exchanges, they are clearing houses that require you deposit coins with them so if they get hacked you lose your coins (Mt. Gox anyone?). Also, they all have registration processes and require either credit cards or an account with a cash-equivalent that requires a credit card (or bank account), thus your transactions are no longer anonymous, there is a digital trail back to you through the credit card/bank account.
So, the question is can you buy bitcoins anonymously? Yes, but its hard without a face-to-face transaction. For example, you could do the following:
(1) create an anonymous email account via a proxy
(2) start a Paxum account (which is anonymous) using the email address
(3) buy a postal money order using cash
(4) mail the money order to Paxum
(5) wait for the funds to be credited
(6) start a Tradehill account using the anonymous email address
(7) transfer funds from Paxum to Tradehill
buy bc with your Tradehill account
(9) delete all three accounts
Obviously a very laborious process. All you hear in BC is accounts accounts accounts, register register register. People don't seem to get that bc users want to be ANONYMOUS. They don't want to register and create accounts. When I go to the grocery store I don't want to be informing Star Market who the hell I am. I just want groceries, not a goddamn account.
Also, the anonymous process above only works for BUYING bc, not selling them. There is no formal way for anonymously selling bc that I can see. For example, if you reverse the Paxum process above they will send you a check, but you need to provide an address and if you cash the check you lose your anonymity at that point since all US banks now require ID or a verified account before cashing a check. You can no longer write checks to "cash" as you used to be able to. The only way around it I see is that you have the check made out to a coin store then hand deliver the check to the coin store and buy gold or silver with it, then sell the gold or silver, an 11-step process.
Obviously there is a need for exchanges to (1) be able support the transfer a coin without taking possession of it, and (2) send and receive anonymous cash payments (manila envelopes anyone?).