I finally figured out what is wrong, and it doesn't appear related to implementing the FeatureLayer class. It seems that if there is only a FeatureLayer in the map, then it won't pay any attention to it unless you explicitly set the map extent. Using data from the ESRI sample services:This loads as expected:
<esri:Map x:Name="MyMap">
<esri:ArcGISTiledMapServiceLayer ID="StreetMapLayer"
Url="http://server.arcgisonline.com/ArcGIS/rest/services/ESRI_StreetMap_World_2D/MapServer"/>
<esri:FeatureLayer ID="MyFeatureLayer"
Url="http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/Specialty/ESRI_StatesCitiesRivers_USA/MapServer/0"/>
</esri:Map>
Nothing loads for this scenario:
<esri:Map x:Name="MyMap">
<esri:FeatureLayer ID="MyFeatureLayer"
Url="http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/Specialty/ESRI_StatesCitiesRivers_USA/MapServer/0"/>
</esri:Map>
However, this does work (which does nothing more than set the map to the extent and spatial ref of the feature layer):
<esri:Map x:Name="MyMap">
<esri:Map.Extent>
<esri:Envelope XMin="-158.064605712891" YMin="19.6961498260498" XMax="-67.986768734036" YMax="64.8691042579708" >
<esri:Envelope.SpatialReference>
<esri:SpatialReference WKID="4326"/>
</esri:Envelope.SpatialReference>
</esri:Envelope>
</esri:Map.Extent>
<esri:FeatureLayer ID="MyFeatureLayer"
Url="http://sampleserver1.arcgisonline.com/ArcGIS/rest/services/Specialty/ESRI_StatesCitiesRivers_USA/MapServer/0"/>
</esri:Map>
This seems like a bug to me.