cogadh: As a developer of Windows-based games, I would expect them (CDProjekt/GOG) to have access to pre-release versions of upcoming Windows versions for the purposes of future game development. That being said, having access to a pre-release version does not guarantee that it can be used to ensure compatibility. Using a beta version might not be useful at all, depending on how much changes between the beta and the final release. Maybe once Windows 7 gets to the release candidate stage it could be used to ensure compatibility, but anything earlier than that would probably be a waste of time.
AndrewC: From what I know devs have to buy they're OS' and don't get any freebies unless they're part of the networks Microsoft distributes software through.
In my experience, nearly all Windows software developers, regardless of the kind of software they develop, are at least part of the Microsoft Developer Network (MSDN) and probably TechNet, too. It would be stupid for a company that develops applications for the Windows platform to not join them and gain access to all its resources, including early OS releases. Heck, I'm part of them and I was only the manager of a department that developed and tested Windows software, I can't program to save my ass.