I think your extent might be off. Try this one:
new esri.geometry.Extent({"xmin":11301731.652648658,"ymin":7041163.49558074,"xmax":11783729.18582064,"ymax":7225388.51426907,"spatialReference":{"wkid":2283}})
An easy way to get a map extent to use in an app is to preview the map service from your services directory, pop open the chrome or firebug console and type:
dojo.toJson(map.extent.toJson())
You can use the value returned by dojo.toJson() as the input to esri.geometry.Extent().
Try this:
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link rel="stylesheet" type="text/css" href="http://serverapi.arcgisonline.com/jsapi/arcgis/2.1/js/dojo/dijit/themes/claro/claro.css">
<style>
html, body { height: 100%; width: 100%; margin: 0; padding: 0; }
</style>
<script type="text/javascript">var djConfig = {parseOnLoad: true};</script>
<script type="text/javascript" src="http://serverapi.arcgisonline.com/jsapi/arcgis/?v=2.1"></script>
<script type="text/javascript">
dojo.require("dijit.layout.BorderContainer");
dojo.require("dijit.layout.ContentPane");
dojo.require("esri.map");
function init() {
var initExt = new esri.geometry.Extent({"xmin":11301731,"ymin":7041163,"xmax":11783729,"ymax":7225388,"spatialReference":{"wkid":2283}});
var map = new esri.Map("map", {extent: initExt});
var basemap = new esri.layers.ArcGISDynamicMapServiceLayer("http://gis.co.frederick.va.us/ArcGIS/rest/services/TestSDEService/MapServer");
map.addLayer(basemap);
}
dojo.addOnLoad(init);
</script>
</head>
<body class="claro">
<div dojotype="dijit.layout.BorderContainer" design="headline" gutters="false" style="width: 100%; height: 100%; margin: 0;">
<div id="map" dojotype="dijit.layout.ContentPane" region="center" style="overflow:hidden;"></div>
</div>
</body>
</html>
