When someone wants to unmount a volume and the mount point is used, he gets a message telling him that the volume can't be unmounted because an application uses it. The user has no idea which application is actually using it and can't remove the device.
I propose that the popup tells him which application(s) is(are) using the device and propose to terminate them (that list should be kept up to date when the app. dies).
Developer comments
Upstream bug (GNOME #528559) has a patch in discussion. Volunteering to work on it.