I think the Solution Explorer window should be smart enough to group partial class files together in the Solution Explorer window; similar to the resx/designer tree it already does for WinForms.
In general, that seems like a good idea, but my specific use case is as follows:
I do a lot of development in C# and have come to love code Region's. They're a good way to organize your code, but honestly I like partial classes more. Instead of putting code into Region chunks, I want to separate chunks of logic out into partial classes and then be able to deal with different aspects of the code in a pure form. There's no problem doing that with todays IDE, other than the excessive amount of .cs files you end up with in the Solution Explorer window. Dumping these files into folders helps, but its ugly.
(I edited this to add this note: in a lot of cases a need for such separation might imply bad design, but in control development its quite handy)