WPF - Combobox with a Prompt

Question

I'm looking for a way to add a "SELECT An Item" to a combobox that does not have an item selected. This would be different than a selected item as a default.

I want the combobox to say "SELECT an Item" and be bound to one list for the possible selections and another model for the selected item.

I'd prefer a style that I can apply to multiple comboboxes and have a way to set the prompt. I've seen something similiar at http://marlongrech.wordpress.com/2008/03/09/input-prompt-support-in-wpf/, but the it does not work smoothly, requiring 2 clicks to get to the list.

Thanks!