We have the Move Implementation to Source File refactoring.
However, all refactorings are reversable and just like there are times when the implementation in a header is getting to large and I want to move it to the source file for single translation unit compilation, there are also times when an implementation is so simple (one-line getter) that I want to move that implementation from the source file to the header file.
Any time VA X lets me perform a refactoring from A->B, I should also be able to apply the inverse refactoring to go from B->A at a later date, i.e. long after I've closed Visual Studio and Undo simply isn't going to work.