With every release of SQL Server, Microsoft has emphasized one area of technology. For SQL Server 7.0, that area was scalability; for SQL Server 2000, it was security. For SQL Server 2005, the emphasis is system and database availability. Microsoft has not only added one completely new technology, database mirroring, to achieve higher availability, but also substantially improved existing availability features.
SQL Server 2005 provides four high-availability technologies: failover clustering and database mirroring, both with supported automatic failover; and log shipping and replication, with either manual or custom-coded failover. Because Microsoft supports automatic failover for both failover clustering and database mirroring, they're clearly the technologies of choice to maximize uptime. If you don't need automatic failover or you're willing to custom-code your automatic failover processes, log shipping and replication might provide the availability you need.