Silverlight question

This is something puzzling. In one of my new project for Windows Phone 7 I am using a Listbox in Silverlight.

 I also handled an event for each time the selected item changed in the Listbox and everything is working well.

At the start of the app I populate the Listbox with some data, and I am selecting the first item. In XAML I have a setter property to change the background of the selected item.

Still good. It works well for any new selected item (click on an item). However I can't figure out how to trigger the SelectionChanged for the deault item, so that I can apply the setter property.

Something I can do so easily on .Net!

Anyone with some ideas? Thanks!