I have been working on standardizing the source control structure for our Team Foundation Server rollout for the new year. I have started by using the Microsoft Team Founda
I like your idea of putting the Sandcastle files as a peer to Source and Tests, I would add a documentation folder, that would then contain the sandcastle files, and optionally the actual documentation.
There are definetly differences of opinions and I'm sure I will be downvoted for this (since I have been before). I would put the generated documentation in TFS for a couple of reasons:
One thing I don't see which I always struggle with is where to third party dependancies, it might be that they belong down under the source so they are with each project although if you wanted to share them accross projects you could add a new top level node.
For my binaries I usually end up with
$/ThirdParty/Company/Product/Version/Src(optional)
So for example I have
$/ThirdParty/ Microsoft/ EntLib/ 3.1 4.0 ComponentArt/ WebUI/ 2008.1/ Src
I like to add the source, I've had to patch CA's source which I hate to do but when a third party doesn't fix the bug you have to resort to this.