I read this article and can think of many uses for enterprise quality encrypted instant messaging on Pocket PC phones.
Mobility Site: WebMessenger Launches Enterprise Instant Messaging for Windows Mobile 5.0 PDAs
The ability to IM a field worker to inform them to synchronize their mobile application is a very good example - "Charlie - you have a new work order!" Charlie pushes the synchronize button on his PDA and downloads the new work order.
Another example - "Charlie, click here for a map to your next job"