Joel Spolsky has good things to say about the Windows source-code tree.
(As always, I speak for myself and not for Microsoft)
There's always talk about how incredibly difficult managing such a hierarchical structure must be and how slowly changes propagate across branches and so on. It's not as bad as it looks because while there is process involved, the most important thing is the product. Important changes are fast-tracked to ensure everyone gets them earlier. Breaking changes (like, say a change to the XML library) are announced throughout the Windows team, making sure everyone calling into it has enough head-time to ensure they're compatible. Anything that'll affect the product quality, usability and development speed is prioritized across branches and source trees. Looking at it as a static, purely process-driven system run by PHBs does it (and the people who make it work) a disservice.
Of course, the system isn't perfect. It's an ongoing process, and we have to see how it can be improved on to make sure we do better and our ship cycles shorten.
Joel link via Ari's blog, who also points to some failings in the system.
Joel also criticizes the 'Shutdown' menu in Windows Vista. I agree grudgingly. On my laptop, the 'Sleep' option is darked out so I can't use it. Fast user switching is great if more than one persion is using the same machine (say, in a family) and some of the ideas Joel gives don't work with existing hardware, or with all of Microsoft's hardware vendors. There's more to these options than meets the eye, and I wish someone on one of these teams would post an explanation which may help us understand these decisions better. I didn't find anything when I looked.