What are the defaults for Binding.Mode=Default for WPF controls?

Similar to UpdateSourceTrigger, the
default value for the Mode property
varies for each property.
User-editable properties such as
TextBox.Text, ComboBox.Text,
MenuItem.IsChecked, etc, have TwoWay
as their default Mode value. To figure
out if the default is TwoWay, look at
the Dependency Property Information
section of the property. If it says
BindsTwoWayByDefault is set to true,
then the default Mode value of the
property is TwoWay. To do it
programmatically, get the property
metadata of the property by calling
GetMetadata and then check the boolean
value of the BindsTwoWayByDefault
property.

Source: https://web.archive.org/web/20100209025938/http://blogs.msdn.com/wpfsdk/archive/2006/10/19/wpf-basic-data-binding-faq.aspx

The safest way would be to always be explicit what kind of binding mode you want from a binding.