Postback doesn't fire when radiobutton is defaulted to checked

I noticed that if I have a set of radio buttons and an updatepanel whose triggers are the CheckedChanged event for those radiobuttons that the postback never fires for the radio button that defaults to checked.  For example:

<asp:Radiobutton id="rdo1" runat="server" text="One" checked="true" />
<asp:Radiobutton id="rdo2" runat="server" text="Two" />
<asp:Radiobutton id="rdo3" runat="server" text="Three" />

If I select Two or Three, they will postback and the UpdatePanel will correctly "rebuild."  However, One (the one that is checked="true") will never fire.  Any thoughts?