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.