I don't think its a bubble, everyone is still very anchored to cheap $2-2.50 corn, $3 wheat, and $6 beans and can't deal with this move as being rational. There was always ample supply but with a growing population and greater demand for meat in the 3rd world, and higher fertilizer and fuel costs, we are never going to see those prices from 2 years ago again. They aren't making more farmland on this planet.
Short term, the grains are overbought, but bull markets often stay overbought for long periods of time.