Vista - Putting default Text into a ComboBox

I've got a combobox I'm trying to style such that if no item is selected,
there is dead text in there ("Select a value..."). However, while I can get
the trigger to properly fire when SelectedValue={x:Null}, setting the Text
property won't work because my combobox isn't editable. This seems like a
very simple template to do, but for some reason, the solution is escaping
me. Anyone know a working way to do this?

FYI: The solution I came up with was to override the combo's style with a 2
column grid. A combobox bound to the source combo spans both columns, and a
textblock with the default text sits over column 1 & is set to visible only
when selectedvalue == null;