SQL Server 2008 R2 has been released. See SQL Server 2008 R2 & What's New (SQL Server 2008 R2) for details about this version.

You may download the Express version from Download details: Microsoft SQL Server 2008 R2 - Express or Download details: Microsoft SQL Server2008 R2 RTM - Express with Advanced Services.

Other versions will be available to MSDN & Technet subscribers May 3rd.