Jimmy May is sharing a very good slide-deck about the importance of Disk Partition Alignment.
You can find some other useful information about this topic on the Microsoft web site:
· Predeployment I/O Best Practices - SQL Server Best Practices Article
· Disk Subsystem Performance Analysis for Windows subsys_perf.doc
· Disk performance may be slower than expected when you use multiple disks in Windows Server 2003, in Windows XP, and in Windows 2000