You have never heard of a heterogenous infrastructure, have you? One very large network I know is a combination of Windows, Sun and Linux.
Apple has also put a lot of effort into making OS X work seamlessly with Windows. Simple workgroup networking between my Mac and my PC is easier than between two PCs.
I'm kind of familiar with heterogenous networks. We've got Windows workstations and back-end servers, some single-purpose *nix boxes for things like document archival and retrieval, and an IBM mainframe. And I'm familiar with the the problems involve in getting it to all work together.
"Simple workgroup networking" between a couple of computers at you home is one thing. When it has to scale to several thousand, and pass an FDIC security audit it gets a little more complicated.