With the war (but not the terrorism) over in Iraq, there is now some uncertainty over where the Sunni Arab refugees are. Many have returned, but the numbers don't add up. The UN has only 200,000 of these refugees registered (and eligible for refugee aid). Since 2003, the UN registered 400,000 Iraqi refugees, most of them Sunni Arabs. Arab nations hosting these refugees believe they still have 1.5 million, although some refugee pundits believe there are only 500,000.
By 2007, most of the five million Iraqi Sunni Arabs in Iraq had been driven from their homes. These people were the power base for Saddam Hussein and his Baath Party. In the previous four years, violence between Sunni Arabs, and the 15 million Shia Arabs, and five million non-Arab Kurds, caused most of this population movement. The cause of all this had been the effort by Sunni Arab terrorists, to regain control of the country. This effort failed, but the numerous attacks on Shia Arabs created a violent backlash.
But there's some odd aspects to the movement of these refugees. Since they're mostly Sunni, it's reasonable that none went to Iran, and equally reasonable that lots went to Jordan (whose Sunni king was always friendly towards Iraqi Sunni Arabs). But some two-thirds of the 1.7 million Sunni Arabs who have left the country ended up in Syria. While Syria is largely Sunni Arab, the country is run by the Shia Alawites, who are close allies with Iran. Meanwhile, Saudi Arabia, which was believed to be covertly backing some of the Sunni militias in Iraq, hasn't admitted any, at least officially. About half the 3.4 million Iraqi Sunni Arab refugees have fled to other parts of Iraq, and many were expected to eventually flee the country.
Syria's Shia ruling class has learned how to deal with their Sunni majority. In addition, the Syrian Shia shares political attitudes with many Iraqi Sunni Arabs. Both countries have a Baath ("renewal") party, and the fall of Saddam has caused the two Baath parties to settle a decades old feud (over which branch of the party was in charge). Nevertheless, this period of peace and reconciliation appears to be over. By 2007, Syria was stopping additional Sunni Arab refugees from entering the country. Jordan was also becoming less hospitable by 2007, and pressure was on Saudi Arabia to let Iraqi Sunnis in. The Saudis feared many of those refugees would be Islamic radicals and terrorists. The Saudis don't want to deal with more of this extremism, even though most of it began in Saudi Arabia. Increasingly, the Saudis don't have much choice.
By 2008, with most Sunni Arab groups in Iraq pledging their loyalty to the government, refugees began to return. But many may never return, because they were Baath Party members, or belonged to Saddam's security forces and had blood on their hands. Many Shia and Kurd vengeance groups had lists of known Saddam supporters, and by 2007 had tracked down and killed thousands of them. Iran provided additional lists, and other support, to Shia death squads. It's still not safe in Iraq for a lot of Sunni Arabs.