The transition according to Microsoft for client and server operating systems are … minor…then major…then minor…etc. That is how server and tools unit president Bob Muglia sees it. Basically, Microsoft is sticking to a schedule presented several years ago. It stated that the server and tools unit would put out alternating minor and major versions every two years. The most recent update of the server version of Windows 7, called Windows Server 2008 R2, was a minor update. This update was completed last year.
Read more here: Will Windows 8 and Server "Next" to Be Developed in Parallel | Windows 8 News