The RC0 version of SQL Server 2008 is available from the Microsoft Download Center. They have Enterprise, Standard, Evaluation and Express Edition available. RC0 will expire after 180 days.
Previously, Microsoft’s Anthony Carabino announced on June 7, that SQL Server 2008 RC0 is available for download on MSDN Subscribers and TechNet Plus Subscribers. I have been playing with RC0 for almost a week, and so far, I am pretty happy. This build is much improved over the February CTP (CTP6).