Microsoft announced at the Microsoft Hardware Engineering Conference (HEC 2008) that the next release of Windows Server — Windows Server 2008 R2, due for release in 2010 — will only be available in 64-bit versions (Intel X64, AMD 64-bit, and Intel Itanium processors).
Windows Server 2008 R2 should not be viewed as a Service Pack (SP). It is a major new release, based on the Windows 7 kernel as opposed to the Windows 6 (Longhorn) kernel utilized by Windows Server 2008. The reason for calling it Windows 2008 R2 as opposed to Windows Server 2010 is that it delivers primarily the same external functionality as Windows Server 2008. In general, it will deliver enhancements to both internal (e.g., number of processors supported) and external functionality (e.g., remote “power shell” access) but not a major new architecture or a new set of APIs, as was the case with Windows Server 2008 (as opposed to 2003).
Before freaking out too much, it is worth noting that Windows Server 2008 R2 (on the Intel X64 and AMD 64-bit processors) will be able to run 32-bit Windows services, though it will require 64-bit drivers. Despite this, Microsoft is encouraging server ISVs to “bite the bullet” and make the leap to 64-bit. This is not simply a case of recompilation, but of rearchitecting server applications to exploit 64-bit capabilities, chief among which is access to much more RAM.
It is worth noting that Microsoft has already followed its own advice with respect to Microsoft Exchange 2007. This is available only as a 64-bit application. It thereby gained many architectural advantages, such as reducing disk I/O more than threefold.