It's always tough to say how agricultural subsidies will pan out with respect to the poor. After all, the poorest people are usually in rural areas, often subsistence farmers. If they're totally subsistence farmers, they don't care about the price of the goods they produce, but if they're doing any selling they need high prices. Price supports keep prices artificially high in the subsidizing country, but they can lead to gluts which are sometimes dumped on developing countries, killing their prices. Thus, subsidies may or may not help the rural poor.
On the other hand, the urban poor have to buy food. Lower prices unambiguously help them, so price supports hurt.
Weighing all the evidence, this extended blog post argues that price distortions and trade barriers on balance hurt the poor. Their simulations show that without the barriers to trade, poverty would drop by 3%. It's a complicated picture, but it makes sense!
