Today Microsoft posted a paper discussing IPSec to the download center. I've been reading it tonight with great interest. You can check it out here:
Internet Protocol Security for Microsoft Windows Server 2003
The Microsoft Windows Server 2003 operating system includes an implementation of the Internet Engineering Task Force's Internet Protocol security (IPSec). IPSec, which is also included in Windows 2000 and Windows XP, provides network managers with a key line of defense in protecting their networks. IPSec exists below the Transport layer, so its security services are transparently inherited by applications. IPSec provides the protections of data integrity, data origin authentication, data confidentiality, and replay protection without having to upgrade applications or train users.