Microsoft confirmed officially that it was developing the next iteration of the Windows client and server operating systems in parallel just as it had done with Windows 7 and Windows Server 2008 R2, and Windows Vista SP1 and Windows Server 2008 before them. However, following his keynote at Tech•Ed 2010, Bob Muglia, president, Server and Tools Business, noted that Microsoft had yet to slap an official codename on the successors of Windows 7 and Windows Server 2008 R2. Muglia also stated that, at this point in time, there wasn’t really a feature list that the Redmond company had been settled on, and that a target release deadline was also up in the air.

"Just like with Windows Server 2008 R2 and Windows 7, we worked on a common schedule and a common thing with our friends in the client team, and we're still doing that," Muglia stated. (via Beyond Binary)

In the past, Steven Sinofsky, president, Windows and Windows Live Division, also objected to the Windows 8 moniker being used for the evolution of Windows client and server. However, despite this, various other Microsoft internal sources are, in fact, referring to the successor of Windows 7 as Windows 8.
Source -
Windows 8 Client and Server Developed in Parallel - Windows 8 Server a major release - Softpedia