Ok, this announcement is a week late, but anyway here goes...
SQL Server 2008 RC0 was released on June 10th. It is available for download for TechNet and MSDN subscribers and also for the public: Download Link. Please make sure you read the Release Notes and Readme, before going ahead with the installation. Plus, there a few things which you may have to look into before doing an upgrade as well.
You may also want to try out the stuff included in the Feature Pack.

Gogula G. Aryalingam (MVP - SQL Server)