Windows Server Division has named the next server Windows Server 2008 R2. The release would be a "minor" or Update release. In a blog posting, Windows Server '7' aka "Windows Server 2008 R2", the team announced that the plan of keeping with a two-year minor release, four-year major release cycle for servers.
The Windows Server 2008 Product Roadmap shows that R2 will release in 2010. It explains that an update release "integrate[s] the previous major release with the latest service pack, selected feature packs, and new functionality."
Group Program Manager Ward Ralston added, "The [Windows] client will be a major release, but, as we’ve said before, compatibility with previous versions of Windows Vista and Windows Server 2008 is a design goal."