Thats true, and there has been some great strides in border security.
The insurgency in Iraq fighting our troops right now are primarily from Iran and Syria. They are trained members of Hamas, Islamic Jihad, etc. who have snuck across the border. I would think that rather than using the Iranian army, the Mullahs in retaliation to a strike against them would try to send as many terrorist militants as possible to sneak into Iraq to join Al-Zarqawi.