It appears for ArcGIS Server 9.x the GetLegendGraphic operation was never implemented. Apparently this operation is an optional implementation and is not a part of the "basic WMS" standard.
From the ESRI docs:
"it is not necessary for a WMS service
to support all the operations. But one
must support at least GetCapabilities
and GetMap operations to be a "Basic
WMS"
The good news however is that in ArcGIS Server 10 this functionality has now been added.
The difference between WMS 1.1.1 and 1.3.0 is two fold.
CHANGE NO 1 - CRS/SRS Usage
Use SRS for 1.1.1
Use CRS for 1.3.0
CHANGE No 2 - WMS 1.3.0 ONLY
The order of parameters for BBOX depends on whether the CRS definition has flipped axes. You will see this in the GetCapabilities request at 1.3.0 - the response should show the flipped axes.
BBOX=xmin,ymin,xmax,ymax NON-FLIPPED
BBOX=ymin,xmin,ymax,xmax FLIPPED
I have made a list of EPSG codes that need to be flipped by creating a SpatiaLite 4.3.0 database and then saving this SQL request to file:
SELECT auth_srid, has_flipped_axes, ref_sys_name, axis_1_name, axis_1_orientation, axis_2_name, axis_2_orientation FROM "spatial_ref_sys_all" WHERE auth_name = "epsg";
You will then see that EPSG:4326 needs to have flipped axes.
4326 1 WGS 84 Latitude North Longitude East
THIS IS THE CORRECTED 1.3.0 REQUEST
Change is BBOX=24,-126,50,-66
http://mesonet.agron.iastate.edu/cgi-bin/mapserv/mapserv?map=/mesonet/www/apps/iemwebsite/data/wms/goes/conus_ir.map&SERVICE=WMS&REQUEST=GetMap&VERSION=1.3.0&WIDTH=256&HEIGHT=256&FORMAT=image/png&TRANSPARENT=TRUE&BBOX=24,-126,50,-66&LAYERS=conus_ir_4km_900913,conus_ir_4km&CRS=EPSG:4326&STYLES&
Best Answer
Try this experimental QGIS plugin: CSI WMS Legend
(On option tab set search for "experimental")
The plugin Displays the GetLegendGraphic image of the selected WMS layer. The plugin has worked with several WMS I've tried. But with the WMS you specify it did not work.