Assuming you have a datalist called "listview1" and a bound field called "DateShipped", the insert template will have a textbox called "DateShippedTextBox".
Top prepopulate it with a value all you need to do is add some code in the form's load event or some other event (Not necessarly the listbox's loading events) and modify the Text property. - THe key is using the "FindControl" method of the listviews insertitem collection.
** Update **
For this to work porperly you need to place the code in the databound event of the listview control
e.g.
C#
protected void listView1_DataBound(Object sender, System.EventArgs e){
TextBox tb;
tb = (TextBox)listView1.InsertItem.FindControl("OrderDateTextBox");
tb = (TextBox)listView1.DateTime.Today.ToShortDateString();
}
VB
protected Sub listView1__DataBound(ByVal sender As Object, ByVal e As System.EventArgs) Handles ListView1.DataBound
dim tb as TextBox= CType(listview1.InsertItem.FindControl("DateShippedTextBox") ,TextBox)
tb.Text =Now().ToShortDateString()
end Sub