Here's another way to approach things. You can provide a control for each item to use as the ToolTip using this strategy:

The Xaml:

<UserControl.Resources>
        <DataTemplate x:Key="ToolTipTemplate">
            <Border BorderThickness="3" CornerRadius="5" 
                    BorderBrush="Azure" Background="AliceBlue">
                <Grid Margin="5">
                    <Grid.RowDefinitions>
                        <RowDefinition />
                        <RowDefinition />
                    </Grid.RowDefinitions>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition />
                        <ColumnDefinition />
                    </Grid.ColumnDefinitions>
                    
                    <TextBlock Text="Name:"
                               Margin="0,0,5,0"/>
                    <TextBlock Text="Quantity:" 
                               Grid.Row="1" 
                               Margin="0,0,5,0" />

                    <TextBlock Text="{Binding TopSalesperson}" 
                               Grid.Column="1" />
                    <TextBlock Text="{Binding Quantity}" 
                               Grid.Row="1" 
                               Grid.Column="1" />
                </Grid>
            </Border>
        </DataTemplate>
    </UserControl.Resources>

    <Grid x:Name="LayoutRoot" Background="White">
        <igMap:XamMap x:Name="theMap" >
            <igMap:XamMap.Layers>
                <igMap:MapLayer x:Name="theLayer" 
                                Imported="theLayer_Imported" 
                                DataMapping="Name = Name; ToolTip = ToolTip">
                    <igMap:MapLayer.Reader>
                        <igMap:ShapeFileReader x:Name="reader" 
                                               Uri="ShapeFiles/usa_st"
                                               DataMapping="Name=STATE_NAME"/>
                    </igMap:MapLayer.Reader>
                </igMap:MapLayer>
            </igMap:XamMap.Layers>
        </igMap:XamMap>
    </Grid>

And the code behind:

public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();

            TestDataItem.Template = 
            (DataTemplate)Resources["ToolTipTemplate"];
        }

        private void theLayer_Imported(
        object sender, 
        Infragistics.Controls.Maps.MapLayerImportEventArgs e)
        {
            MapLayer layer = sender as MapLayer;
            if (layer == null)
            {
                return;
            }
            if (e.Action != MapLayerImportAction.End)
            {
                return;
            }

            layer.DataSource = new TestData();
        }
    }

    public class TestData
        : ObservableCollection<TestDataItem>
    {
        public TestData()
        {
            Add(new TestDataItem()
            {
                Name = "New Jersey",
                ToolTipValue =
                new OtherData()
                {
                    TopSalesperson = "Jessie",
                    Quantity = 300
                }
            });
        }
    }

    public class OtherData
    {
        public string TopSalesperson { get; set; }
        public int Quantity { get; set; }
    }

    public class TestDataItem
    {
        internal static DataTemplate Template { get; set; }

        public string Name { get; set; }
        public object ToolTipValue { get; set; }
        public object ToolTip
        {
            get
            {
                DataTemplate template = Template;
                ContentControl control = new ContentControl();
                control.ContentTemplate = template;
                control.Content = ToolTipValue;
                return control;
            }
        }
    }

Hope this helps!

-Graham

This looks like a great way of going after it. Only one question: Where is "TestDataItem" defined?

Its the last section of code in the code behind portion of the post.