public class NetworkState : INotifyPropertyChanged
    {
        public NetworkState()
        {
            NetworkChange.NetworkAddressChanged +=
                   (sender, args) => SendNotifyPropertyChanged("IsOnline");
        }

        public void SendNotifyPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }

        public bool IsOnline
        {
            get
            {
                return NetworkInterface.GetIsNetworkAvailable(); ;
            }
        }
        
        #region INotifyPropertyChanged Members

        public event PropertyChangedEventHandler PropertyChanged;

        #endregion
    }

<UserControl x:Class="SilverlightNetworkState.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    xmlns:local="clr-namespace:SilverlightNetworkState"
    Width="400" Height="300">
    <UserControl.Resources>
        <local:NetworkState x:Key="State" />
    </UserControl.Resources>
    <Grid x:Name="LayoutRoot" Background="White">
        <CheckBox Margin="10" IsChecked="{Binding Path=IsOnline, Source={StaticResource State}}" Content="Online" />
    </Grid>
</UserControl>