One of our more common technical questions at IMS is "What is a good tool to disguise voices in recordings?" In the most recent VMN, one of our field workers provides a quick overview of his experiences with the commercially-available Ircam Trax v3 VST plugin in Adobe Audition CS6 to alter and disguise voices. Read the article in VMN #89 section 4 for more details.
Update: The article says that you must purchase an iLok licensing dongle ($50) in order to install the plugin. With the current version of the plugin, if you have a good internet connection, you only need to have an iLok account, which is free.