In short, NO it will not void the warranty. Upon a part of the terms of the
Magnuson-Moss Warranty Act. Magnuson-Moss Warranty Improvement Act states, a vehicle manufacturer may not make its vehicle warranty conditional on the use of any brand of security system unless the manufacturer provides the system free of charge or the Federal Trade Commission (FTC) has specifically published that only the vehicle manufacturer’s product may be used. To challenge a false claim, ask the person to put it in writing, or request the vehicle manufacturer’s security system free of charge. If you are charged for the security system, or they refuse to give you a written statement, there may be a violation of Federal law.
In many cases they claim that it voids the warranty as a scare tactic to have you buy the equiptment through them. Which if they don't have an OEM add-on piece, they will just install an aftermarket system anyway, and probably charge you more to do it.
And I have witnessed the dealerships/mechanics try blame aftermarket alarms/remote starts for problems that may occur without looking for the problem. Usually once they see it installed, they turn around and blame the product/install and charge you for the full diagnostic. For example, I had a guy come into my shop stating the mechanic said that the system we installed was to blame for the ignition problems he was having with his car not running properly. But what the mechanic didn't realize is the system we installed was only a simple aftermarket keyless entry system, and had nothing to do with the ignition system. We still looked it over, but told the guy to get his money back. Several times we inform the customer that our system did not cause the problem, and instruct them to have the stealership/mechanic to call us directly and explain the problem in detail. Usually we never hear from them again, but if we do we find that the technician was either talking out of his ass, or didn't do a proper diagnostic of the problem.