Microsoft's Scott Schnoll has just posted a new blogcast demo of the Cluster Continuous Replication (CCR) feature in Exchange 2007. Simply put, CCR is native log shipping for Exchange. Although it requires the Microsoft Cluster Service, it is unlike traditional Shared Copy Clustering (SCC) because it doesn't require the clustered machines to share storage (which means a SAN or iSCSI solution). Instead, transaction logs are replicated across an SMB fileshare and used on the second machine to reconstitute the database.
Scott knows a hell of a lot about Exchange, and has been doing a great job of helping the Exchange 2007 early adopters and reviewers get up to speed on CCR (and Exchange 2007 in general); take a few minutes and watch his blogcast.