Service Pack 2 for Windows 7 and Windows Server 2008 R2 has already actively been in development since fall 2010. If all goes according to plan, it is reportedly expected to arrive in mid-2012.
Windows 8, meanwhile, is still in the milestone development phase; M3 development will commence in March 2011. The OS is scheduled for the very end of 2012, and the server version will be named Windows Server 2012. Microsoft is reportedly planning two betas for Windows 8 and at least one will be available for public consumption. Three months after the RC is released, Windows 8 will hit the Release to Manufacturing (RTM) milestone. Last but not least, Microsoft plans to begin selling boxed retail versions of Windows 8 on Monday January 7, 2013.
While the roadmap details could certainly be on target, Microsoft's internal plans do change and nothing is set in stone. Furthermore, exact dates leaked this early are even less likely to be accurate. We've known for a while that Microsoft is targeting 2012 for the release of Windows 8.
It doesn't make sense for the operating system to arrive in January; this happened with Vista and the software giant was very sorry it missed holiday 2006 sales. It made sure to get Windows 7 out in time for holiday 2009. We expect Redmond will do everything in its power to get Windows 8 out in time for holiday 2012.