Well actually, there’s none at the moment, but a there is a knowledgebase article available on SP2 as a place holder for future information:

Beta Information

This article discusses a beta release of a Microsoft product. The information in this article is provided as-is and is subject to change without notice.

No formal product support is available from Microsoft for this beta product. For information about how to obtain support for a beta release, see the documentation that is included with the beta product files, or check the Web location where you downloaded the release.

INTRODUCTION

This Microsoft Knowledge Base article will be updated with more information about Windows Server 2008 Service Pack 2 (SP2) and Windows Vista Service Pack 2 (SP2) when the information becomes available. Currently, the product release notes and related information about Windows Server 2008 SP2 and Windows Vista SP2 are not available.