At present, there is not an equivalent to IMxDocument.CurrentLocation. You will need to capture the position within a mouse event such as the following, using the ClientToMap method:
Just to clarify there is no IMxDocument, but there is the MapView class which might give you what you need. If you use MapView.Active you will get the current active MapView, or null if there is no Map View open. Once you have the active MapView you can look at the Camera property to get the location in 2D where the camera defines its viewing location using X and Y. This will be the center point of the 2D view’s extent.
Hope this helps.