I programatically change a bound TextBox value, but the value does not get pushed back into the bound datasource. How can I make sure the DataSource is updated

You can call the EndCurrentEdit method on the bindingmanager for the TextBox.

[C#]
	this.textBox1.Text = 'XXXX';   //set the value
	this.textBox1.DataBindings['Text'].BindingManagerBase.EndCurrentEdit();  //end the edit

[VB.NET]
	Me.TextBox1.Text = 'XXXX'
	Me.TextBox1.DataBindings('Text').BindingManagerBase.EndCurrentEdit()