You can project the point using a GeometryService. Note that you will need to convert your coordinates to decimal degrees before projecting them.
There is also a good sample that shows how to do this:
var inSR = new esri.SpatialReference({
wkid: incoord
});
var outSR = new esri.SpatialReference({
wkid: outcoord
});
var geometryService = new esri.tasks.GeometryService("http://tasks.arcgisonline.com/ArcGIS/rest/services/Geometry/GeometryServer");
var inputpoint = new esri.geometry.Point(inlon, inlat, inSR);
var PrjParams = new esri.tasks.ProjectParameters();
PrjParams.geometries = [inputpoint];
PrjParams.outSR = outSR;
if (datumtrans != 'Default') {
PrjParams.transformation = {
wkid: parseInt(datumtrans, 10)
};
}
geometryService.project(PrjParams, function (outputpoint) {
console.log('Conversion completed. Input SR: ' + incoord + '. Output SR: ' + outcoord + '. Datum Transformation: ' + datumtrans + '.');
dojo.byId("outLat").value = outputpoint[0].y;
dojo.byId("outLon").value = outputpoint[0].x;
});