As per the MSDN page, you can specify a query string in the format
<uriMapper:UriMapping Uri="/Products/{type}"
MappedUri="/Views/ProductDetail.xaml?producttype={type}">
</uriMapper:UriMapping>
I do not know how to link the type to a value through XAML but on navigation to that page, you can add an OnClick event instead of a navigateuri. In the OnClick event you will specify something like the following:
private void Link2_Click(object sender, RoutedEventArgs e)
{
Uri x = new Uri(String.Format(/Products/{0},yourcombo.SelectedItem), UriKind.Relative);
//ContentFrame is the Navigation Frame
ContentFrame.Navigate(x);
}
This will navigate to the ProductDetail.xaml page. From here you can get the producttype value by using string type = this.NavigationContext.QueryString["producttype"];
Tim Heuer also has an excellent web cast on the navigation solutions.