It turns out, there's one more member of the Colorado family that's in the pipeline: "Breckenridge." Here's a mention of Breckenridge from a privacy statement on the Microsoft Connect download site, and additional info on Windows Server Codenamed Breckenridge is extremely scarce.
It appears that this Windows Server operating system is related to the next major iteration of Windows Home Server. Microsoft's currently cooking Windows Home Server Codenamed "Vail," or the second major version of its home sever.
However, Codenamed Breckenridge isn't exactly that close to Vail. Instead it's being built to function more like a storage server, than to offer the diversified capabilities of a WHS release.
And in this sense, the consumers segment targeted will be completely different, namely business customers rather than home users.
Of course, with no actual details available from Microsoft, this all needs to be taken with a grain of salt.