Bitcoin-qt app stores private keys in ~/Library/Application Support/Bitcoin/wallet.dat
Make sure you encrypt your wallet (Settings -> Encrypt Wallet). Write down the password on a secret piece of paper ;-)
Make sure you back it up properly. By default, Time Machine should backup entire Application Support, but this would exhaust your backup disk quickly because it will continuously back up 3 GB blockchain. I recommend excluding every Bitcoin/* file except wallet.dat (see the screenshot).
Remember: if you lose wallet.dat, your money is gone forever.