Edu's argument is knocking down a strawman. It is an unfair characterization of the vegetarian position. Of course no one is advocating chopping down rainforests for soy. That's not what they want.
It is obvious that the environmental cost / calorie produced goes up as you move up the food chain. That is to say, a calorie of beef costs way more to the environment than a calorie of soy (or corn or grass, but we cannot eat grass). This should be evident from the fact that cows have to grow skeletons to support their meat, and we don't eat those bones. So what vegetarians, at least the sensible non-moral version i support, are advocating is to eat closer to the bottom of the food chain to reduce environmental cost. Instead of the acres and acres of fields that are producing corn in iowa for animal feeds, plant some plants there! perhaps soy, or rice, just something we would eat. In this sense, eating less meat reduces the environmental cost.
Think about it, rainforests could be just as well chopped down to produce things we feed cows. and in that case, it is even less inefficient.
Of course, i actually think all monoculture agriculture is bad. And you should never plant just one crop, but instead use some kind of natural foodchain to produce multiple crops (and animals) on a farm. But since our current paradigm is dominated by industrial monoculture farms, then i'd rather those monoculture farms produce things for humans to eat, instead of for the animals human eat to eat -- for the sake of environmental cost.
Note: this is Sam's argument, though posted by Edu