Once a wallet file is initially created, does the reference client generate additional private keys?
When a wallet is created, it contains 100 keys in its "key pool". Only one of them is made visible, but when you create a new address, it actually just takes one key from the pool. The reason for this is so you don't need to backup your wallet.dat file after every transaction or new address,

New keys are generated every time one is pulled from the key pool, if possible, to keep the key pool at size 100. In an encrypted wallet, the passphrase is required to store new keys, so the key pool is only filled when the wallet gets unlocked.