Hyper-V Server 2008 is RTM. You can download it as of October 1st.

Microsoft Hyper-V Server 2008 provides a simplified, reliable, and optimized virtualization solution, enabling improved server utilization and reduced costs. Since Hyper-V Server is a dedicated stand-alone product, which contains only the Windows Hypervisor, Windows Server driver model and virtualization components, it provides a small footprint and minimal overhead. It easily plugs into customers’ existing IT environments, leveraging their existing patching, provisioning, management, support tools, processes, and skills.

You can find all info on Hyper-V Server here.