Just wanted to post a quick note that we have released Windows Server 2008 Release Candidate 0!
You can see the official announcement here and will be able to find the download and read more about it here. The download page itself is currently being updated and it says that "Windows Server 2008 Release Candidate 0 will be available here shortly". So keep checking!
If you wanted to test Exchange 2007 SP1 on Windows Server 2008, this is a great time to do it!
EDIT: One more thing that I wanted to add quick is - if you want to know how to test Windows Server Virtualization with this RC 0 build, here is how to install it.