Note that Bitcoin addresses and transactions are inherently public, and the only thing that's private (and the only thing which needs protecting with encryption) are the private keys used to sign transactions. Anyone who knows your addresses can see how many bitcoins you have, where they came from, and where they're going. Privacy is only achieved by the difficulty of associating Bitcoin addresses with real-life identities. You can encrypt your entire wallet using Truecrypt or EncFS if you really want to, but be aware that this might not provide as much protection as you think.
Just because some punk found a wallet.dat on my machine doesn't mean he needs to know what my private keys are, and thus how much the wallet is worth. He won't be able to touch them, but he was able to breach my privacy by snooping on my personal finance. If I had considerable funds, that may motivate him to install a keylogger on my machine or something.
The standard wallet encryption encrypts your private keys, and only your private keys. Your private keys are
not necessary to tell how many bitcoins you have, since all transactions are public. There is no financial privacy (in the traditional sense of the term) when using Bitcoin. This is the price you have to pay for a zero-trust payment system. Since there is no bank or other trusted third party to verify transactions and account balances while keeping them secret, everyone needs to be able to see everyone else's transactions to prevent people from fraudulantly spending money they don't have. To protect your privacy, you need to ensure that your identity cannot be linked to your bitcoin addresses.
It is most unlikely that you will specifically be targetted by hackers if you have a considerable amount of bitcoins. You more likely to be targetted just for having Bitcoin installed at all, in order to get whatever little money you have. However, the chances of this happening are very low. In fact, I don't think anyone has ever had bitcoins stolen as a result of a keylogger being used to get the passphrase to a stolen wallet.dat file using the standard wallet encryption (though it's not impossible, and additional encryption is useless againsts a keylogger).
I was planning on using just the standard wallet encryption before backing up my wallet.dat to "the cloud", but having learned this, I'll wrap it in an additional layer. Thanks for the clarification, guys (and your great work, Gavin).
A good idea, just remember that encrypting your wallet isn't the whole story if you're worried about your privacy rather than just having your money stolen.
You can encrypt your entire wallet using Truecrypt or EncFS if you really want to, but be aware that this might not provide as much protection as you think.
What is this supposed to mean? I thought truecrypt was impossible to break if you have a long enough pass?
Correct. However, the OP's question indicates that he is concerned about keeping has balance secret, which is something that encryption won't help with if his identity can be linked to his addresses some way other than his wallet file.