We "were" in the same boat: our company decided to build UIs using C# and leaving the hardcore modules to C/C++, interfacing them via COM. Developing UI interfaces in C++ under Windows is a pain in the ass.
An example of this is our SpyStudio product[1], which was developed in C# using our Deviare technology[2] for instrumenting binary applications.
We had a legacy application using MFC/C++ and no one anymore likes MFC at the studio, so we had to either somehow switch to C#/Windows.Forms|WPF overnight or slowly move to something else. Since there is MfcMigration toolkit for Qt - slow progress was the choice - so every week less and less of MFC is visible, and more and more from Qt.
Other toolkits can benefit if there is a transition layer - or at least something where you can plug into the existing message pipeline, windowing system - and then start replacing widget by widget, dialog by dialog.
An example of this is our SpyStudio product[1], which was developed in C# using our Deviare technology[2] for instrumenting binary applications.
[1] http://www.nektra.com/products/spystudio-api-monitor/
[2] http://www.nektra.com/products/deviare-api-hook-windows/