If you are using the ESRI Flex Viewer, my widget should be able to help you out. If you are not using the ESRI FLex Viewer you can use the source as an example.
Thanks for Your perfect widget. I have one problem with accuracy in tranformation form "S_JTSK Krovak" (102067) to "WGS84" (4326). It seems taht tranformation uses default method with accuracy appr. 100 meters. Is it possible to implement some better tranformation methods. For example In ArcMap is possibility to set it when You load layer to MXD.