You may now download the Windows HPC Server 2008 release candidate via http://connect.microsoft.com. Register for the "Windows HPC Server 2008" beta program. You'll also want to grab the corresponding SDK. Please do provide usability feedback using the "Feedback Tool" on the beta program web-site.
For technical discussions, join and participate in the Windows HPC forums accessible via the "Forums" tab on http://windowshpc.net.