Well, the matrix is a little more straightforward than that.

XP won't get 9 and 10. Vista won't get 10 IE 6 is being EOL'd, most new applications don't support it.

So, you need to support XP w/7,8; Vista with 7,8,9, and Windows 7 with 7,8,9,10.

But, (And, not being a web developer, I'm very curious about this) - does the operating system really matter that much - Doesn't IE 7 perform reasonably simliar on XP, Vista and WIndows 7? Is it the security model that makes most of the difference? Or the included version of Java? Where do operating systems have the largest impact on Web Development?