From this:

WASHINGTON -- After blunt U.S. diplomatic warnings, Iran and Syria have at least temporarily closed their borders with Iraq in moves that could prevent hundreds of Al Qaeda and Ansar al Islam extremists from escaping into Iran and cut off Arab fighters and military equipment arriving from Syria, according to senior U.S. officials...

To drive home its message with Iran, U.S. officials in recent weeks have held at least two rounds of secret talks in Europe with Iranian representatives and relayed three "tests" through Swiss diplomats in Tehran, designed to determine whether the Islamic republic intends to use the Iraq war to back away from or draw closer to terrorists, U.S. officials say...

"We don't want Iran to make the same mistake as it did in Afghanistan, where there was an understanding about going forward in a cooperative way in looking at the future," said another senior administration official. "They allowed or extracted Al Qaeda from Afghanistan..."

The article also discusses Hezbollah, Supreme Council of the Islamic Revolution of Iraq, etc. See also "U.S.: Iran will infiltrate 5 Iraqi cities."