>>>>Example:
>>>>The default UpdateSourceTrigger for a TextBox is LostFocus(). If I leave it that way and somewhere in code execute: TextBox.Text="Hello World" this is not relayed back to the source - until (and if) the TextBox subsequently loses focus.
>>>>To updated the source immediately I need to set the UpdateSourceTrigger to PropertyChanged. If I do that then the source is also updated on every keystroke.
>>>
>>>Wouldn't TextBox.Text="Hello World" remove the binding on TextBox.Text and replace it with the string "Hello World"? Since the binding on TextBox.Text is now gone, the source would never be updated.
>>>
>>>Or am I still not following?
>>
>>Following completely - and I agree entirely that directly setting the Text property should replace the binding. The trouble is that is not what I'm seeing. It's been a long day and I may be missing something obvious but it you have time try this:
>>Code:

using System;
>>using System.ComponentModel;
>>using System.Diagnostics;
>>using System.Windows;
>>using System.Windows.Data;
>>
>>namespace BindingTest
>>{
>>    public partial class MainWindow : Window, INotifyPropertyChanged
>>    {
>>        public MainWindow()
>>        {
>>            InitializeComponent();
>>            DataContext = this;
>>        }
>>
>>        string theText;
>>        public string TheText
>>        {
>>            get { return theText; }
>>            set
>>            {
>>                if (theText != value)
>>                {
>>                    theText = value;
>>                    OnPropertyChanged("TheText");
>>                }
>>            }
>>        }
>>
>>        public event PropertyChangedEventHandler PropertyChanged;
>>
>>        void OnPropertyChanged(string s)
>>        {
>>            if (PropertyChanged != null)
>>                PropertyChanged(this, new PropertyChangedEventArgs(s));
>>        }
>>
>>        private string s = "String A";
>>        private void SetTarget(object sender, RoutedEventArgs e)
>>        {
>>            TB.Text = s;
>>            s = s == "String A" ? "String B" : "String A";
>>        }
>>    }
>>
>>    public class TBConverter : IValueConverter
>>    {
>>        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
>>        {
>>            if (value == null) return Binding.DoNothing;
>>            return ((string)value).ToUpper();
>>        }
>>
>>        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
>>        {
>>            return ((String)value).ToLower();
>>        }
>>    }
>>}

XAML:

<Window x:Class="BindingTest.MainWindow"
>>        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
>>        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
>>        Title="MainWindow" Height="350" Width="525" 
>>        xmlns:local="clr-namespace:BindingTest">
>>    <Window.Resources>
>>        <local:TBConverter x:Key="TBC" />
>>    </Window.Resources>
>>    <StackPanel>
>>        <TextBox x:Name="TB" Height="31" HorizontalAlignment="Left" Margin="71,45,0,0"  VerticalAlignment="Top" Width="148"
>>                 Text="{Binding Path=TheText,Mode=TwoWay, Converter={StaticResource TBC}}"/>
>>        <TextBox x:Name="TB2" Height="31" HorizontalAlignment="Left" Margin="71,45,0,0"  VerticalAlignment="Top" Width="148"
>>                 Text="{Binding Path=TheText,Mode=TwoWay, Converter={StaticResource TBC}}"/>
>>        <Button Click="SetTarget">Set Target</Button>
>>    </StackPanel>
>></Window>

The binding and converter still appear to be operating after 'TB.Text = s;' (you need to set focus to the TextBox and tab out to get the source updated or, better, just set the UpdateSourceTrigger to PropertyChanged)
>
>Oh, wow, you're right, that does work. The binding doesn't get replaced when it's a TwoWay binding. It does break if it's OneWay. (Good to know)

OneWayToSource doesn't break either. Of course, if it did, then it would be a pretty pointless option (s). In that light the behaviour when Mode is TwoWay makes sense.

One other small discovery : If I set Mode to OneWayToSource with the above converter in place then the designer throws a NullReferenceException and refuses to display. The fix was to add a check for null in the ConvertBack() which was being called by the IDE