Styrofoam is made by a combination of air and polystyrene. The two are molded into cells and pressed together. The main component of styrofoam is the chemical styrene. Polystyrene itself is non-toxic to our bodies; but it can cause an inflammatory response when enclosed within us. Depolymerization can occur at higher temperatures; when this happens chain scission may occur and monomers can be released. Polymers, at low temperatures are highly preferred in most cases, but at higher temperatures you may approach the ceiling temperature.
The ceiling temperature is where polymerization and depolymerization are at equilibrium with each other. Styrofoam is not FDA-approved inherently but can be manufactured in such a way so that it is. Styrofoam with a high ceiling temperature is better used and much less dangerous. The average microwave cannot reach a temperature high enough to hit the ceiling temperature. The ceiling temperature of polystyrene is three hundred ninety-five degrees celsius. The polymer would already have began charring at this temperature; in other words, you would need to make the polystyrene very very hot in order to release such monomers.
Short term exposure to the chemical styrene can cause stomachaches or irritation/inflammation of your eyes and nose. If you are exposed to this chemical at high temperatures for a longer period of time you may experience headaches, fatigue, depression, hearing loss, nervous system disfunction, or other symptoms. The temperature at which styrofoam is dangerous to be around is much higher than the maximum temperature that a microwave can reach. Certain forms of polystyrene will react differently at different temperatures but I would say do not place styrofoam in a microwave just to be safe. Unless it is written that the styrofoam has been tested and is microwave safe, just use a different material to heat up your food.
Sources:
http://theboard.byu.edu/questions/213/
http://www.reddit.com/r/askscience/comments/1bjzxo/can_you_microwave_styrofoam_is_it_safe_to_eat/