I hope this isn't a stupid question.

Scenario

Standard Bitcoin client with several receiving addresses in it. I made a backup of the wallet.dat file. I continue to use bitcoin and over time(say a week) I have received several coins. The transactions show different receiving addresses when I get these coins that don't show on the receiving addresses tab in the client. Do a small bet on satoshidice and win or lose you get something back to an address newly created to receive and that address is not in my receiving addresses list.

My question is with the above scenario where I lose my wallet.dat due to hd crash or whatever and I restore from my backup thats a week old. What happens to the auto-generated receiving addresses that were in use post backup? How does the restored wallet.dat know those are my coins? How would a backup wallet a week old be able to pull all that back together? Its my understanding that each address you can receive bitcoins on must have a matching private key entry in your wallet.dat so you can use those coins. So how could a backup even be useful if new coins coming in get new addresses?