As this blog entry says, although the command line interface of the ESXi server (the virtualization host platform of the VMWare vSphere) isn't derived from Linux, but it is an independently developed, proprietary product.
There is a common misconception, that ESXi is a Linux, because its command line interface is very similar. It is true, but in the deeps there is a kernel which is completely different from the Linux.
But, this kernel, the VMKernel, uses a lot of different Linux drivers ported into it. Of course, its developers doesn't bored to develop independent drivers, they have simply imported them from the already available Linux kernel sources.
Thus, the driver modules of the VMKernel are Linux-derived products, thus they must be released on GPLv2. And, as I know, on the GPLv2 is it possible to sell software, but the owner must also release its source code.
Having the source code of the VMKernel-ported drivers would make also possible to develop VMKernel drivers independently.
But, as I know, no part of the VMKernel was released in source form until now.
How is it possible? Or I know something badly?