Ilya Shapiro and Caitlyn W. McCarthy, both of the Cato Institute, have a short paper in the John Marshall Law Review on the Citizens United case, addressing in particular the issue of whether corporations should count as real persons for First Amendment purposes.
Reviewing the Supreme Courts treatment of the constitutional rights of corporations in a variety of contexts, Shapiro and McCarthy convincingly show that the Court has never thought that corporations have constitutional rights in exactly the same way as individual human beings do. On the contrary, the Court has consistently recognized that corporations, though not identical to human beings for constitutional purposes, should be accorded some constitutional rights in order to protect the constitutional rights of the individuals who manage and own them.
For example, corporations clearly have Fourth Amendment rights against illegal searches and seizures and Fifth Amendment rights against government takings. If they didnt, the police could storm corporate offices whenever they wanted and seize whatever they pleased (Fourth Amendment), and governments could take corporate property without paying compensation or even giving the corporation a hearing (Fifth Amendment)and these outcomes would clearly impair the constitutional rights of the individuals who manage and own the corporation. On the other hand, corporations do not have Fifth Amendment rights against self-incrimination, even though the individual human beings who are corporate officers, directors, or shareholders do.
The real issue in Citizens United is thus not whether corporations have constitutional rights just like individual human beings do (of course, they do not), but whether giving effective protection to the First Amendment rights of the human beings who own and manage corporations requires recognizing such rights in the corporation itself. For more on that question, see not only the paper by Shapiro and McCarthy, but also this blog by UCLA law professor Eugene Volokh.