UserControl and Two-way Binding

Hello,
 I want to create a UserControl with a TextBox and a Button inside it.  The UserControl's DataContext must bind to the TextBox's Text property.  When the bound value changes, the TextBox must reflect that change (change towards the control) and when the TextBox is updated by the user, the bound value must change (change towards the bound value).  My understanding is that the XAML below should be enough to make this work.  However, the second direction (towards the bound value) does not work.  The first direction works well.

Here is my UserControl:

<

UserControl x:Class="Billabong.FolderPicker" xmlns=http://schemas.microsoft.com/winfx/2006/xaml/presentationxmlns:x=http://schemas.microsoft.com/winfx/2006/xamlxmlns:s="clr-namespace:System;assembly=mscorlib">
<StackPanel Orientation="Horizontal">
<TextBoxText="{Binding . ,Mode=TwoWay}"/>
<Button>O..</Button>
</StackPanel>
</UserControl>Here is an example of it's usage from another XAML Window:

<bb:FolderPicker DataContext="{Binding Path=SomeStringProperty}">
</bb:FolderPicker>

How to I get the TextBox in my User Control to send updates to the bound object?  Do I really have to write a DependancyProperty for this?

Regards,
Willem