If you had funds sent to the deposit address on your account on an exchange, how likely is it the account could be discovered to be yours if someone was watching the transaction closely? If you buy darkcoin for instance, then sell quickly is it still possible to trace to your account on the exchange?
most of the exchanges have your personal info and certainly they share that info with various authorities. You can then use this data, perhaps correlate it with other databases to determine the ownership of Bitcoin accounts. I'm sure it's very easy to correlate a Bitcoin address to a gmail account or facebook account and these days that means they might as well have your passport.
Darkcoin does not have much to their claims about a privacy implementation.
-bm