I have a GET request which looks like this ("Hotspot" is a valid layer name):
GET /arcgis/services/SmallvilleOGC/MapServer/WMSServer?SERVICE=WMS&map=SmallvilleOGC&VERSION=1.1.1&REQUEST=GetMap&FORMAT=image/png&SRS=EPSG:4326&BBOX=-82.6,34.7,-82.2,35.1&WIDTH=1024&HEIGHT=1024&TRANSPARENT=TRUE&LAYERS=Hotspot&STYLES= HTTP/1.1
This is a mechanism to make sure that the layers can be requested with a unique identifier. By using the layer order, there will be no duplicate layer numbers, and no chance for confusion, since it is possible to publish a map that has multiple layers with the same name. In ArcGIS 10, there will be an option available to use the layer name if there are no duplicate names.