Citizens United vs. Federal Election Committee is the case Carol C. refers to, I believe.
Do corporations have a right to speak on political questions?

One argument to be presented is that corporations are individuals under the constitution, and should be able to express their views.

Soliciter General Elena Kagan is expected to argue that U. S. election law has been built on the notion that corporations and unions cannot use their general fund to defeat a candidate because the individual shareolders and union members may not want their money spent that way, and because the large amount of money would skew and corrupt the system.
Mostly quoted from-

http://www.pbs.org/newshour/updates/law/july-dec09/scotus_09-09.html

It will be an interesting case.