Liberal groups, like Think Progress, are jumping all over Mitt Romney for this (via TP):
Mitt Romney completed a rowdy campaign stop at the Iowa state fair, before a key Republican debate tonight and an upcoming Iowa straw poll. At the end of his speech, a Q&A session quickly devolved into a shouting match during which he defended the rich, argued for cutting entitlements, and equated corporations with people. Romney told a group of angry Iowans that raising the retirement age to protect corporate tax breaks is appropriate. “Corporations are people, my friend,” he said.
Now I don’t think Romney was making a legal argument about corporate personhood, which is well established concept in US law:
In the United States, corporations were recognized as having rights to contract, and to have those contracts honored the same as contracts entered into by natural persons, in Dartmouth College v. Woodward, decided in 1819. In the 1886 case Santa Clara County v. Southern Pacific Railroad, 118 U.S. 394, the Supreme Court recognized that corporations were recognized as persons for purposes of the Fourteenth Amendment
Rather, I am pretty sure he was trying to say that corporations are made up of people, but not in a Soylent Green sort of way. Rather they are comprised of workers generating goods and services for customers. And when you punish corporations, you punish workers and shareholders and customers. A few additional points:
1) Here an interesting bit from an OECD paper on taxes and economic growth
Corporate taxes are found to be most harmful for growth, followed by personal income taxes, and then consumption taxes. … A second option is to reform corporate taxes, as they influence productivity in several ways. Evidence in this study suggests that lowering statutory corporate tax rates can lead to particularly large productivity gains in firms that are dynamic and profitable, i.e. those that can make the largest contribution to GDP growth. It also appears that corporate taxes adversely influence productivity in all firms except in young and small firms since these firms are often not very profitable. … Lower corporate and labour taxes may also encourage inbound foreign direct investment, which has been found to increase productivity of resident firms. In addition, multinational enterprises are attracted by tax systems that are stable and predictable, and which are administered in an efficient and transparent manner.
2) And here is economist Greg Mankiw addressing the topic in his popular economics textbook:
Many economists believe that workers and customers bear much of the burden of the corporate income tax. To see why, consider an example. Suppose that the U.S. government decides to raise the tax on the income earned by car companies. At first, this tax hurts the owners of the car companies, who receive less profit. But over time, these owners will respond to the tax. Because producing cars is less profitable, they invest less in building new car factories. Instead, they invest their wealth in other ways—for example, by buying larger houses or by building factories in other industries or other countries. With fewer car factories, the supply of cars declines, as does the demand for autoworkers. Thus, a tax on corporations making cars causes the price of cars to rise and the wages of autoworkers to fall.
The corporate income tax shows how dangerous the flypaper theory of tax incidence can be. The corporate income tax is popular in part because it appears to be paid by rich corporations. Yet those who bear the ultimate burden of the tax—the customers and workers of corporations—are often not rich. If the true incidence of the corporate tax were more widely known, this tax might be less popular among voters.
3) Finally, economists Kevin Hassett and Aparna Mathur on who bears the burden of corporate taxes: “The results in this paper suggest that corporate tax rates affect wage levels across countries. Higher corporate taxes lead to lower wages. A 1 percent increase in corporate tax rates is associated with nearly a 1 percent drop in wage rates.”

Thomas Paine said it best in The Rights Of Man in 1791.
“It has been thought that government is a compact between those who govern and those who are governed; but this cannot be true, because it is putting the effect before the cause; for as man must have existed before governments existed, there necessarily was a time when governments did not exist, and consequently there could originally exist no governors to form such a compact with. The fact therefore must be, that the individuals themselves, each in his own personal and sovereign right, entered into a compact with each other to produce a government: and this is the only mode in which governments have a right to arise, and the only principle on which they have a right to exist.”
Thomas Paine and others of the Revolutionary Era realized that any institution made up by and of humans – from governments to churches to corporations – must be subordinate to individual living people in terms of the rights and powers held by the institution.
Corporations only gained equal status with people after decades of assault on the Constitution by the railroads in the 1800′s. The peak year for their legal assault was 1877, with four different cases reaching the Supreme Court in which the railroads argued that governments could not regulate their fees or activities, or tax them in differing ways, because governments can’t interfere to such an extent in the lives of “persons” and because different laws and taxes in different states and counties represented illegal discrimination against the persons of the railroads under the Fourteenth Amendment.
In 1886 the Supreme Court ruled on an obscure tax issue in the case Santa Clara County vs. Union Pacific Railroad, but the Recorder of the court, a man named J. C. Bancroft Davis, himself formerly the president of a small railroad wrote into his personal commentary of the case that the Chief Justice had said that all the Justices agreed that corporations are persons. This, in fact, was not true at all.
In so doing, he – not the Supreme Court, but its clerical recorder – inserted a statement that would change history and give corporations enormous powers that were not granted by Congress, not granted by the voters, and not even granted by the Supreme Court. Davis’s headnote had no legal standing, but was taken as precedent by generations of jurists, including the Supreme Court who followed and read the headnote but not the decision.
The Founders never intended corporations to have the same rights as citizens. It doesn’t matter how many rationalizations the right wing think tanks disseminate to their armies of bloggers and pundits. It is only because of an obscure headnote written by a corrupt Supreme Court clerk in an obscure railroad tax case that took place in 1886 that they have been able to excercise such power, and to the detriment of the People.