Yesterday, even as the UN Security Council unanimously passed new sanctions on Iran (the third round of sanctions on Iran over its weapons program), President Ahmadinejad visited the US-controlled Green Zone in Baghdad, met with Iraqi leaders, and called the "brotherly ties" between Iraq and Iran unbreakable. Such a visit gives legitimacy to an essentially US-created government in Iraq, and improves the likelihood that the US-supported Iraqi government will succeed. Such actions on the part of the Iranian leadership indicate a possible shift in disposition with respect to the United States and the Middle East.
Iran, unsurprisingly, fully denounced the UN sanctions against them, insisting that their nuclear program is peaceful, and that they will move forward with it. The US-supported sanctions are somewhat confusing; the US National Intelligence Estimate of late 2007 insisted that Iran had dismantled its weapons program in 2004, and it appeared that the United States and Iran had cut a deal; it is possible that these sanctions are a sign that relations are deteriorating.
Nonetheless, the Ahmadinejad visit did happen; it was the first Iranian presidential visit to Iraq since 1979, and shows a new shift towards a closer relationship. The visit likely shows that Iran is buying into the government, if not directly in its current form, then into what it hopes the government will become. US suspicions of Iranian military meddling in Iraq have been quiet for quite some time; it is possible the Iranians are more worried about an unstable Iraq than a secular or pro-western one.
Ahmadinejad made important and token gestures against Israel and the United States while visiting. He denounced Israeli action in the Gaza Strip, as well as a continued US presence in Iraq. Both Iraqi and Iranian officials made it clear that US troops were not used for security of the meeting building (even if the meeting was in the US Green Zone); essentially the meeting was planned and executed as if the US was not a major force in Iraq. Ahmadinejad spoke to the US as hurting Iraq with its continued occupation, but also that Iran and Iraq should help each other and stay close.
Iran may be positioning itself to act as a paternal figure in Iraq after the US leaves, or is simply looking forward to a relationship with the 25 million-person country in a post-Saddam Hussein era. Ultimately, the Iranian leadership's anger with the West and the United States may be great, but it is likely not enough that Iran would see more interest in trying to disrupt the Iraqi government simply for the sake of hurting American policy; that, or Iran recognizes that a more successful Iraqi government is likely lead to a more expedient US withdrawal.
In a final note, increased Iraqi security operations during Ahmedinijad's visit were unable to prevent two suicide attacks in Baghdad, which killed 19.