Here's an interesting bit of analysis. There's a summary here:http://www.huffingtonpost.com/huff-wires/20070505/un-organic-food and more at the FAO site:http://www.fao.org/organicag/ofs/docs_en.htm Thanks to Aaron for pointing me to this one!
The gist of the thing is this - even allowing for a dramatic fall in yield from organic agriculture (which is by no means inevitable), poor farmers would be better off using organic techniques for two important reasons. The first is that reduction in costs would offset any loss in yields for individual farmers - that is, they might be growing less food, but they'd also be spending much less of their limited funds to grow it, and would have more money to spend on food. The second is that the soil damage and other consequences would be sufficiently less that their long term food security would be enhanced by organic agriculture.
Now there are some problems with these models - one of them is the assumption that in a world with vastly more organic agriculture, that organics would still command higher than average prices. But it is worth noting that the UMichigan study suggests we can feed the whole world without Monsanto, ADM and Cargill.