The following code will take a screenshot of whatever is in the backbuffer at the time it is called:
Public Sub SaveScreenshot(ByVal filename As String, Optional ByVal fileformat As ImageFileFormat = ImageFileFormat.Bmp)
Dim backbuffer As Surface = VideoDevice.GetBackBuffer(0, 0, BackBufferType.Mono)
SurfaceLoader.Save(filename, fileformat, backbuffer)
...which means that Managed DirectX is extracting the back buffer to system memory, converting the surface to a GDI+ image, and exporting the image to a file.
I'm fine with that, and I support the functionality. So, if Managed DirectX can handle that, why won't it let me extract out the back buffer to system memory so that I can do some intense back buffer effects?