Hi Johnny,I ran a simple test using a proxy and an ArcGIS.com basemap and was able to add your wms layer at both 3.2 and 3.3. I know that your scenario is quite different in that it sounds like you have a basemap in the same Spatial Reference as the wms layer and you aren't using the proxy. I did a quick search and can't find a test basemap in the spatial reference you are using. Could you modify the test code below so that it reproduces the issue?
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=7,IE=9">
<!--The viewport meta tag is used to improve the presentation and behavior of the samples
on iOS devices-->
<meta name="viewport" content="initial-scale=1, maximum-scale=1,user-scalable=no">
<title>Map with WMS</title>
<link rel="stylesheet" href="http://serverapi.arcgisonline.com/jsapi/arcgis/3.3/js/dojo/dijit/themes/claro/claro.css">
<link rel="stylesheet" href="http://serverapi.arcgisonline.com/jsapi/arcgis/3.3/js/esri/css/esri.css">
<style>
html, body { height: 100%; width: 100%; margin: 0; padding: 0; }
</style>
<script>var dojoConfig = {parseOnLoad: true};</script>
<script src="http://serverapi.arcgisonline.com/jsapi/arcgis/3.3/"></script>
<script>
dojo.require("dijit.layout.BorderContainer");
dojo.require("dijit.layout.ContentPane");
dojo.require("esri.map");
dojo.require("esri.layers.wms");
var map;
var wmsLayer;
function init() {
map = new esri.Map("map",{
extent: new esri.geometry.Extent({"xmin":-89257.57663483504,"ymin":6286122.683888868,"xmax":1196107.491008342,"ymax":6908625.842243175,"spatialReference":{"wkid":102100}})
});
esri.config.defaults.io.proxyUrl = "http://localhost/proxy/proxy.ashx";
var basemap = new esri.layers.ArcGISTiledMapServiceLayer("http://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer");
dojo.connect(map, "onLoad", function(){
wmsLayer = new esri.layers.WMSLayer("http://grb.agiv.be/geodiensten/raadpleegdiensten/GRB/wms");
wmsLayer.setVisibleLayers([0,1]);
wmsLayer.setImageFormat("png");
map.addLayer(wmsLayer);
});
map.addLayer(basemap);
}
dojo.ready(init);
</script>
</head>
<body class="claro">
<div id="content"
data-dojo-type="dijit.layout.BorderContainer"
data-dojo-props="design:'headline', gutters:false"
style="width: 100%; height: 100%; margin: 0;">
<div id="map"
data-dojo-type="dijit.layout.ContentPane"
data-dojo-props="region:'center'"
style="overflow:hidden;">
</div>
</div>
</body>
</html>
