We have an Open Source Software Lab in Redmond - they have an interesting web site at http://port25.technet.com. It makes a good story to portray Microsoft and the open source world as engaged in a full blooded fight to the death; but life is rarely that simple. A lot of Microsoft customers have some open source software (and Vice versa), as well as wanting to understand what we're competing with, we want things to work well for our customers.
We'd rather people made IE their browser of choice. But if they're going to run Firefox, we don't want them to have a rotten experience of their Windows system as a result. And we'd certainly prefer them to use firefox on Vista than use it on XP or something even older. No sense in detering people from upgrading because the browser they happen to prefer doesn't work so well on the new OS.
I also read a great post on the IE team blog this evening, they've set out a list of things that are fixed in CSS support, but the key stuff is at the end - there is tons of information around IE 7, you can start at the Information Index for IE 7.
A couple of things I'd pick out are the IE 7 Readiness Toolkit and the Checklists for Developers IT professionals and consumers if you support web clients or servers, this tells you where you should concentrate your efforts.