dbus-send --print-reply --dest=com.nokia.HildonDesktop.AppMgr /com/nokia/HildonDesktop/AppMgr com.nokia.HildonDesktop.AppMgr.LaunchApplication string:"mediaplayer"

for the benefit of those looking for an answer, here is a generic solution that'll work for any application that hildon recognizes. you don't even need to know the path of the binary