I've seen Citizens United come up in a couple different places today during Hobby Lobby discussions. Thought it would be useful to re-look at that very briefly.
Citizens United did not establish corporate personhood or declare that corporations are people. That was established a very long time ago (at least as long ago as 1816) and gives corporations the ability to do things like buy and sell property, open bank accounts, enter into contracts, and sue and be sued, among other things. Since then courts have been demarcating the boundaries of this legal fiction by deciding what rights, responsibilities and regulations are applicable to corporations.
Until very recently the general rule of thumb was that corporations didn't get the Constitutional rights given to actual people (e.g., corporations do not get the 5th Amendment protection against self-incrimination). Of course, the exception proves the rule and corporations do get 14th Amendment protection. Justices Black and Douglas were both very careful throughout their tenure on the Supreme Court to note that corporate personhood is not an all or nothing deal - whether a corporation is treated as a person is determined by the factors like the purpose of the regulation and the underlying rights of the individuals who make up the corporation.
So what does Citizens United do if not declare that corporations are people too? It gives political speech (free speech) protection to corporatate, labor unions and not-for-profit spending on independent communications. It doesn't affect the Tillman Act (regulating corporate campaign donations) or campaign contribution limits.
Edited to add: Citizens United is not decided on corporate personhood grounds.