Ward Ralston, a group product manager at Microsoft said on the Windows Server team blog that
Some IT shops still on Windows Server 2000 may also note that extended support for that version of the software ends on July 10, 2010. At the same time, Windows Server 2003 and Windows Server 2003 R2 will move from mainstream support to their extended support phase.
Several years ago Microsoft had committed to a schedule of major and minor releases and this release fits with that schedule, according to Ralston. He also offered that some of the speculation about what is or isn't Windows 7 comes from the code name itself, which is a name used by internal development.
Though Windows Server 2008 R2 is a mid-term release, Ralston said the client version of Windows 7 will be a major release.