How does the Bitcoin Standard Client wallet encryption work?
You only need the passphrase to do certain things like send bitcoins or change the passphrase. 

The encryption itself is attached to the wallet.dat file which contains 


keypairs for each of your addresses
transactions done from/to your addresses
user preferences
default key
reserve keys
accounts
a version number
Key pool


and if you ever want to switch your wallet or add the same wallet to a different computer, this is what you would actually be making a copy of. And because this is the file that's encrypted your passphrase will move along with it. 

You do not need a password to make a copy of it, but making a copy of it, doesn't give you access to the coins. Unless your using a hosted wallet, then you need to make copy of your wallet yourself. Put them on a flash drive or another computer, or maybe even in a safety deposit box.