Oh wait, I just read this paragraph from https://en.bitcoin.it/wiki/Wallet_encryption
"When a wallet is encrypted, the passphrase is required to top up the keypool, thus, if the passphrase is rarely entered, it is possible that keypool might run out. In this case, the default key will be used as the target for payouts for mining, and calls to getnewaddress and getaccount address will return an error. In order to prevent such cases, the keypool is automatically refilled when walletpassphrase is called with a correct passphrase and when topupkeypool is called (while the wallet is unlocked). Note that the keypool continues to be topped up on various occasions when a new key from pool is used and the wallet is unlocked (or unencrypted)."
Okay, that seems like what is happening to me. My wallet is encrypted, but I rarely use my passphrase because I am just receiving Bitcoins at this point. So I suppose that I won't be able to really see what's going on until I enter my passphrase. I suppose that this is all the more reason to switch to a HD wallet!
"When a wallet is encrypted, the passphrase is required to top up the keypool, thus, if the passphrase is rarely entered, it is possible that keypool might run out. In this case, the default key will be used as the target for payouts for mining, and calls to getnewaddress and getaccount address will return an error. In order to prevent such cases, the keypool is automatically refilled when walletpassphrase is called with a correct passphrase and when topupkeypool is called (while the wallet is unlocked). Note that the keypool continues to be topped up on various occasions when a new key from pool is used and the wallet is unlocked (or unencrypted)."
Okay, that seems like what is happening to me. My wallet is encrypted, but I rarely use my passphrase because I am just receiving Bitcoins at this point. So I suppose that I won't be able to really see what's going on until I enter my passphrase. I suppose that this is all the more reason to switch to a HD wallet!