Microsoft Knowledge Base Article 821839 contains the following summary:
This article discusses the use of Internet Protocol security (IPSec) on Exchange 2003 back-end servers that are running on a Windows Server 2003-based
server cluster.
Microsoft supports Exchange 2003 running on Windows Server 2003-based computers and using IPSec transport mode Encapsulating Security Payload (ESP)
to encrypt communication with clustered Exchange 2003 servers that use Network Load Balancing clusters or server clusters. When you use IPSec between
front-end servers and back-end servers, failover times depend on Exchange 2003 recovery time plus the time it takes for IPSec to renegotiate security
associations during the failover process.
