How I now think the keypool works:
 - wallet.dat contains 100 private/public key pairs
 - key pairs are associated with your addresses as well as some (but not all?) transactions
 - when a new address is created or a transaction occurs to/ from an address a key is pulled "from the bottom" of the key pool, with remaining keys left to be used at a future date — and a new key repopulates "at the top"
 - my wallet balance is based on private keys for both addresses and transactions, and then verified against the block chain
 - restoring from a backup will maintain my current balance, so long as fewer than 100 actions have taken place since the backup (address creation and transactions)
 - the keypool can be made larger (in some way that I haven't figured out how to do on my mac yet)