iWork '09 has a new feature that lets you password protect your documents. To protect a document, open the Inspector and then select the Document tab if you aren't already on it. At the very bottom you can check a box next to "Require password to open" and your document will be password protected. Of course you'll want to also hit the "Change Password" button right below that so that you can set the password to be used for the iWork '09 document.