You can take a look at this SDK sample: http://help.arcgis.com/en/webapi/silverlight/samples/start.htm#CustomSymbols. Specifically this code, you can use a different element here and set to a different attribute field, depending on the service.
<esri:SimpleRenderer x:Key="MapTipLabelMarkerRendderer">
<esri:SimpleRenderer.Symbol>
<esri:MarkerSymbol OffsetX="6" OffsetY="6">
<esri:MarkerSymbol.ControlTemplate>
<ControlTemplate>
<Grid>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="MouseOver">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetName="labelOnHover"
Storyboard.TargetProperty="Visibility"
Duration="0">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
<DoubleAnimation From="0" To="1" Storyboard.TargetName="labelOnHover"
Storyboard.TargetProperty="Opacity"
Duration="0:0:.25" />
</Storyboard>
</VisualState>
<VisualState x:Name="Normal" />
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<!--Marker-->
<Ellipse Width="12" Height="12" Fill="Red"
HorizontalAlignment="Left" VerticalAlignment="Top" />
<!--Label-->
<Grid Margin="8,-8,0,0" x:Name="labelOnHover" Visibility="Collapsed"
HorizontalAlignment="Left" VerticalAlignment="Top" >
<!--Text halo using a white blurred text-->
<TextBlock Foreground="White" FontWeight="Bold" Text="{Binding Attributes[AREANAME]}" >
<TextBlock.Effect>
<BlurEffect Radius="5" />
</TextBlock.Effect>
</TextBlock>
<!--Text-->
<TextBlock Foreground="Black" FontWeight="Bold" Text="{Binding Attributes[AREANAME]}" />
</Grid>
</Grid>
</ControlTemplate>
</esri:MarkerSymbol.ControlTemplate>
</esri:MarkerSymbol>
</esri:SimpleRenderer.Symbol>
</esri:SimpleRenderer>
