You may not have read this article before: http://moishelettvin.blogspot.com/2006/11/windows-shutdown-c...

Here's a relevant quote from the article:

    In Windows, this [centralised] model breaks down simply 
    because there are far too many developers to access one 
    central repository. So Windows has a tree of 
    repositories: developers check in to the nodes, and 
    periodically the changes in the nodes are integrated up 
    one level in the hierarchy. At a different periodicity, 
    changes are integrated down the tree from the root to the 
    nodes. In Windows, the node I was working on was 4 levels
    removed from the root. The periodicity of integration 
    decayed exponentially and unpredictably as you approached
    the root so it ended up that it took between 1 and 3 
    months for my code to get to the root node, and some 
    multiple of that for it to reach the other nodes. It should
    be noted too that the only common ancestor that my team, 
    the shell team, and the kernel team shared was the root.

With a distributed system, it would be possible to send changes directly between the two groups. You might have a problem keeping the whole Windows source code repository stored locally on your development machine though.