Like all Windows NT-based operating systems, Windows 8 will be a derivative of the Windows 7 kernel and the Windows Server 2008 R2 kernel. What this means is that by the time Windows 8 is released, it will include significant enough functionality to justify an upgrade. Progress is measured in nightly builds, or compilations of the software, using compilers, as well as key objectives for the development staff. While Windows 8 may currently resemble Windows 7, this is because it is nearly a year away from a commercial release date. It is also simultaneously being designed for ARM processors, which used an entirely different instruction set. These processors function on cell phones, tablets, and other portable devices. For example, your phone may well have a "C" drive by 2014.
Windows 8 will continue to be developed, but the current version released to developers is to help them build programs, drivers, and business models before it ever becomes available to the public. Windows 8 could be "skinned" to look like Windows Classic, or Windows 98, if the developers wanted to. What they are focusing on right now as far as all of the new fancy user interface features are not being shown. It is clear from my evaluation of multiple software builds that certain new features are intentionally being hidden from the public, at least temporarily.
It is very likely that you will see multiple releases of Windows 8, all using a modified kernel. You need to understand that the kernel is what is being developed here, and it is what helps all components of the computer communicate with Windows and vice versa. To understand the development process, be aware that Windows Vista looked almost exactly like Windows XP until very late builds began to leak to the Internet underground. Further, Windows XP "Luna" theme was never seen by anyone when the OS was leaked to the public. The theme that we recognize as being that of Windows XP was thrown into the OS right before its release.
To further understand what is going on, the development team has likely been segmented into groups, working on different parts of the operating system. Look up compilers, operating system development, and so forth and you will understand what I mean.
Having beta tested the Vista RC1 and later both beta and Release Candidate builds for Windows 7(32bit and 64bit) I have to agree with the explaination there. Each new version does require some place to start off from on the initial stages much like the early Alpha stages for Windows 7 were starting to materialize shortly after Vista's release back in 2007.
At first you would only notice a few minor items during the early development appearing as if you were still running the present not the next version to come. By the time the first available public beta release is seen most of the kernel and main gui have already been through most of the major changes to be seen while some last minutes items will still be worked on in preparation for the RTM release(RTM=Release To Manufacturing).
One thing I just reviewed about the next version to come will be it's "portability" as far as the corporate concerns according to one report of seeing W8 running on a usb flash drive as well as being a lot further geared for Tablet PC, netbooks, and other hand held portable devices as well as increased touch screen support.