Why consider a central nervous system to be a necessary condition? It's certainly integral to our experience of pain, but I think to generalize universally from that shows a lack of imagination.

Sci-fi is full of artificial intelligences which lack anything that we'd recognize as a nervous system, but whose internal information-states still bear something analogous to ours. Can we say for sure that something similar is not at work within a plant? We already know that some plants communicate with their neighbours using pheromones. The question is not about their anatomical similarity to us, but about the presence or absence of higher-order experience them - whatever physical form that may take.

All that aside, mijopo's objections above are all quite true. None of this implies that it's unacceptable to eat plants; the point is just that it's problematic to go around affirming or denying creatures' rights based on how much or little they resemble us.