I think that everybody had their doubts when they read that the next Windows version will be called 7. Mostly because the development team said that it will be an evolution of Vista's codebase (that has the kernel version 6.0).
It seems that in fact, the next Windows version will have it's kernel version at 6.1 not 7, so compatibility wise it should be a much smoother update (similar to the one from 2000 to XP).
Here's the official explanation, like it or hate it it's your choice.
Here's where things get a little more complicated. Following Windows 3.0 was Windows NT which was code versioned as Windows 3.1. Then came Windows 95, which was code versioned as Windows 4.0. Then, Windows 98, 98 SE and Windows Millennium each shipped as 4.0.1998, 4.10.2222, and 4.90.3000, respectively. So we're counting all 9x versions as being 4.0.
Windows 2000 code was 5.0 and then we shipped Windows XP as 5.1, even though it was a major release we didn't' want to change code version numbers to maximize application compatibility.
That brings us to Windows Vista, which is 6.0. So we see Windows 7 as our next logical significant release and 7th in the family of Windows releases.