Diya87,
It help to understand how the XY in the Banner is getting there. Basicly when the mouse moves over a Flash Stage the mouse knows its coordinates based on the stage size and values like X: 300 and Y: 126 are returned from the mouse move event, those values are then feed to the map.toMapFromStage function and actual map coordinates are returned. There is not function on the map component that will return Z. As the Flex ArcGIS API is right now and the REST endpoint that Flex consumes there is no Z-Aware capability on a map service. The only way you could possibly get a Z Value from a DEM is to build a geoprocessing task that you could fire on a mouse click. I do not have any examples of how to do this.