Windows 7 & Windows Server 2008 R2 & Windows NT 7.0

“Okay, I don’t know how someone on our side could have miscommunicated this or if you are purposely reporting this incorrectly, but let’s be clear on this: Windows 7 Server is and has always been Windows Server 2008 R2.
“Furthermore, Windows 7, despite it’s rather pretentious sounding code name (a result of Sinofsky’s like of big round numbers) is NOT Windows NT 7, but rather 6.1(current builds are numbered 67xx as a direct continuation of the longhorn codebase). Put simply, it is not a big jump as a codebase revision and the new changes, on both the client and server, will be focused on user features, not core OS components. The big core OS changes are WDDM 2 and a kernel scheduler update to remove the simple bitmask enumeration of processors so that the OS can schedule more than 64 concurrent threads.
“Finally, and I can’t be more clear on this, ‘Windows 7′ client and Windows Server 2008 R2 will RTM simultaneously (and just so you are 100% clear on this) and are based on exactly the same codebase (just as Vista SP1 and Server 2008 are based on an identical core OS codebase).
“As for the next major release (meaning a full revision of the NT codebase) that will not occur until well after the current Win7 wave.”

More info at ZDNet