Yingqi Tang, a product engineer specializing in ArcGIS Server and OGC services, contributed the following post:
In ArcGIS Server 9.2 you can publish a map document as an OGC WMS service. All WMS services advertise the spatial reference systems they support through capabilities files. WMS clients read the capabilities file to find out which spatial reference systems they can use to request a map.
The WMS services you create with ArcGIS Server support hundreds of spatial reference systems, but only two are advertised in the capabilities file: EPSG 4326 (required by every WMS service) and the spatial reference of the data frame from which you published the service. Although you can request a map from the WMS service using any of the hundreds of supported spatial reference systems, some WMS clients will return an error if the spatial reference you request does not appear in the capabilities file. Other clients, such as ArcMap, only allow you to switch among the spatial reference systems listed in the capabilities file.
This post shows how to add more spatial reference systems to the capabilities file of a WMS service that you have published with ArcGIS Server.