Microsoft is sticking to their promise of delivering a minor release after a major Windows Release. Today Microsoft showed a roadmap of Windows Server releases:
Windows Server 2008 (year: 2007)
Windows Home Server (year: 2007)
Windows Server, codename "Centro" (year: 2008)
Windows Server, codename "Cougar" (year: 2008)
Windows Storage Server "Longhorn" (year: 2008)
Windows Server 2008 R2 (year: 2009)
Windows Server 2008 is the last 32-bit edition and release of Windows. So the R2 release will be released as 64 bit only.