Since Saudi Arbia is officially our ally, we can just ignore the Saudi/Iraq border in terms of security? I don't think I can agree with that.
Nor can I agree that the sort of border infiltration that terrorists attempt was wholly prevented under Saddam's regime.
I am aware that since September 2004 the US in concert with Iraqi trainees have been working on border security (
source). A full year later, Zarqawi loyalists took over Qaim (
source). So again, I don't really agree with your implication that sealing Iraq's border is a simple task, but I do at least agree that it could have been worked on sooner. I just don't think it would have had as major an impact on the insurgency as you seem to, though I'm sure I could be wrong.
Anyway, thank you for finally answering the question.