I've solved it. iUI can't show the default symbol.
When I changed the symbol to for example
var sym = new esri.symbol.SimpleMarkerSymbol(esri.symbol.SimpleMarkerSymbol.STYLE_CIRCLE, 12, esri.symbol.SimpleLineSymbol(), new dojo.Color("red") );
map.graphics.add(new esri.Graphic(esri.geometry.geographicToWebMercator(pt), sym, ...
the marker will be displayed.