from http://developers.facebook.com/guidelines.php

IV. Applications displayed on the Facebook Site

Applications that may be displayed on "user profile" pages or other pages of the Facebook Site (i.e., as opposed to pages served by a developer or other third party) may not include advertisements or paid promotional content of any kind, or any web search functionality of any kind. However, these restrictions do not apply to any page primarily dedicated to a developer application (even if framed or hosted by Facebook).