today via their Customer Preview Program. It's avaliable to anyone interested in testing SP2 for Windows Vista and/or Windows Server 2008 prior to the final release.
You can download the Windows Vista and Windows Server 2008 Service Pack 2 RC from the Microsoft Customer Preview Program site or install through Windows Update using their Windows Update Experience Kit. Remember, you will have to uninstall previous versions of the Service Pack before installing the RC and you will have to uninstall the RC version before installing the final version.

Full story: techfragments.com