EDIT: so, could I just encrypt wallet.dat, and then save in dropbox. I was reading over the article. I'd need to redo this regularly, or find a way to automatically encrypt wallet.dat, and then set dropbox to save it regularly. I'm thinking... 
From reading the code, it would seem that by default, a new wallet contains a pool of 100 unused pre-computed private keys for receiving addresses. If you back up an empty wallet, use some receiving addresses (but less than 100), and then restore the empty wallet over your used wallet, bitcoin will still have your balances, although the names attached to the receiving addresses will be lost.