<p>For a Silverlight 2 webapp. I added a combobox. I have an IEnumerable as Itemsource to populate the combobox. Works fine.</p>

<p>But I would like to add an extra item ("please select a....") to the combobox, anyone an idea how this can be done using the Silverlight 2 combobox.</p>

<p>Any more info about using a template for the ComboxboxItems is welcome as well.</p>

You can easily insert an item at a desired index location in the Items collection of the ComboBox using the following code.
TextBlock t = new TextBlock();
            t.Text = "Please select....";
            combo.Items.Insert(0, t);
Setting the selected index will set the ComboBox to show your added item by default:
combo.SelectedIndex = 0;