For agriculture in colder, higher latitudes—such as Minnesota and Northern Europe—the downsides of global warming (extreme heat, drought, flooding, salinization of water supplies, soil erosion, and so on) will quickly outweigh any benefit from increased CO2.
Update 5-16-12 TS: A recent studies showed short term gains in plant productivity under conditions of global warming were lost as the gains diminished over nine years, due to complex interactions with soil, nutrients and other plants. Here's EOM's summary: "New NSF-funded research finds an initial boost, but then plants run out of nutrients. One scientist said Nature doesn't operate like short term tests in a greenhouse. But we're all living in a greenhouse and the experiment is all around us."http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate1486.html
The steroid analogy has been borne out by studies.
Increasing CO2 for plants is like giving athletes anabolic steroids: it may produce short-term weight gain, but often with long-term, negative side effects.