ESRI has released the WMS Layer sample code at the code gallery. You can download it here.

Another layer suppor for the ArcGIS Silverlight API that people requests often.

What’s WMS? From Open GeoSpatial

The OpenGIS® Web Map Service Interface Standard (WMS) provides a simple HTTP interface for requesting geo-registered map images from one or more distributed geospatial databases. A WMS request defines the geographic layer(s) and area of interest to be processed. The response to the request is one or more geo-registered map images (returned as JPEG, PNG, etc) that can be displayed in a browser application. The interface also supports the ability to specify whether the returned images should be transparent so that layers from multiple servers can be combined or not.

How to use it? Add the layer inside the Map Layers like any other.

<esriWMS:WMSMapServiceLayer ID="AGSWMSLayer" 
                     Url="http://sampleserver1.arcgisonline.com/ArcGIS/services/Specialty/ESRI_StatesCitiesRivers_USA/MapServer/WMSServer"                     
                     SkipGetCapabilities="True"
                     Layers="2" Version="1.3" Opacity="0.7"/>

Cheers

Al