The Exchange Team Blog has quite a good overview of the new Standby Continuous Replication (SCR) feature in Exchange 2007 SP1.
Standby continuous replication (SCR) is a new feature being introduced in Service Pack 1 for Microsoft Exchange Server 2007. As its name implies, SCR is designed for scenarios that use standby recovery servers. SCR extends the existing continuous replication features and enables new data availability scenarios for Exchange 2007 Mailbox servers. SCR uses the same log shipping and replay technology as local continuous replication (LCR) and cluster continuous replication (CCR) to provide added deployment options and configurations.
Check out the full post for some great info.