UPDATE: Here is some more behind-the-scenes detail as well as speculation about Hashimi's vote. The author suggests that Hashimi is trying to position himself as a nationalist but is actually following a line that benefits Kurdish interests. He breaks down some of the seat numbers and explains why the Kurds are also interested in the minorities clause (it has to do with increasing Kurdish influence over Shabak and Yezidi lists).
Last week, the Iraqi parliament passed an elections law that was to have resolved some of the contentious issues surrounding voter eligibility. But today, one of Iraq's two vice presidents, Tariq al-Hashimi, vetoed the bill. Hashimi belongs to the Iraqi Islamic Party, a Sunni Islamist coalition. He objected specifically to some of the details of the proposed legislation which limited the representation of "minorities" and Iraqi refugees living abroad to 5%, according to the New York Times. Most of the 2 million Iraqi refugees residing outside the country are Sunnis; their numbers constitute 8% of the country's population of 25 million.
Initially, it seemed that the bill's handling of Kirkuk voter lists - it decided that 2009 lists of city inhabitants would be used - favored Kurdish interests, since it is widely believed that the Kurdish share of Kirkuk's population has increased significantly in the last 5 years. But on Tuesday, the president of the Kurdistan Regional Government, Massoud Barzani, expressed his opposition to the law, threatening a Kurdish boycott in response to the seat allocation (i.e., the 5% limit). Apparently, Iraq's president, Jalal Talabani, who is a Kurd, had also threatened to veto the bill, already before Hashimi did so (NYT).