Over the past couple of years, both the Windows client and Windows server teams have been structuring their releases to alternate between major and minor ones.
(On the server side, the Softies have been rolling out a major release followed by a minor update (known as Release 2, or R2) every two years. On the client side, the timing has been off, but the major/minor cadence has been pretty similar.)
Starting with Windows 7, however, that logic and naming structure that Microsoft has worked to establish for Windows seems to breaking down. In the August 18 posting to the Windows Server blog about Windows Server 2008 R2/Windows 7 Server, Group Product Manager Ward Ralston noted that even though Windows 7 Server (a k a Windows Server 2008 R2) is an interim, more minor release, “the (Windows 7) client in fact will be a major release.”