The best way to define its fungibility is that 1 bitcoin will always be equal to another bitcoin. All the speculations and calculations about how 1 bitcoin is equal to 283 USD etc is determining its price, not its value. So in terms of value, 1 btc = 1 btc always. Regarding the anonymity of bitcoin, yes bitcoins cannot be tracked as they are exchanged between private keys, but the transaction is always recorded publicly.
The key word here is that 1 bitcoin will always be equal to ANOTHER bitcoin. So its not 1 btc = 1 btc, but 1 btc = ANOTHER btc.
And this might not always remain true for all bitcoins. For example, the value of bitcoins that has been to a certain known address might be auctioned off as some sort of an antique item in the future. Perhaps the first address Satoshi spends from after he goes public. Who knows.
I don't think I really believe there is a Satoshi out there, I think that is just a fake name by a smart man who chooses to be anonymous because he is known to have too much power, known to be too rich. That is certainly a threat to his life as he has individualistic power with bitcoins, not like other rich people who keep their money in assets and banks. Either Satoshi is one person or a group of smart mathematicians, now that's a mystery.