We just completed this whitepaper and it'll be available on Technet soon... but I thought I'd share it on my blog as well (and first!)

The document covers both Exchange 2003 Service Pack 2 and Exchange 2007 as well as both Windows Mobile 5.0 and 6! 

It covers the three main components of the Windows Mobile Security architecture:

•       Security on the Device describing the code and features resident on a Windows Mobile device
•       Security on the Exchange Server providing details about the ActiveSync protocol and the security policies that can be managed from Exchange Server
•       Security on the Network outlining the features of a secure mobile messaging network, including ISA Server, Internet Information Services server, and protocols for data encryption and device authentication

This document supports the IT Professional in a large enterprise corporation who has the resources and the mission to deploy a highly secure, controlled mobile messaging solution in his or her existing corporate network configuration.