A similar case to net neutrality has already been ruled in the past, always from wikipedia:
* The first case to use the idea was the Supreme Court's judgment in United States v. Terminal Railroad Association, 224 U.S. 383 (1912). A group of railroads controlling all railway bridges and switching yards into and out of St. Louis prevented competing railway companies from offering transportation to and through that destination. The court held it to be an illegal restraint of trade.
* Associated Press v. United States, 326 U.S. 1 (1945), in which the Supreme Court found that the Associated Press bylaws which limited membership and therefore access to copyrighted news services violated the Sherman Act.
* In Lorain Journal Co. v. United States, 342 U.S. 143, 146-49 (1951), the Lorain Journal was the only local business doing news and advertisements in town. The case was that refusing to place an ad for the customers of a small radio station was a Sherman Act violation. In the end, the court accepted an offer to simply accept the advertisements.
If you hold a facility that can't be easily duplicated you are not really allowed to do as you please with it but you need to offer service to everybody equally or you'll violate the Sherman Act[2].
[1]: http://en.wikipedia.org/wiki/Essential_facilities_doctrine [2]: http://en.wikipedia.org/wiki/Sherman_Antitrust_Act