I always like to see a little more science in articles that use the word "science" in the title.
Here's another take: http://www.second-opinions.co.uk/vegetarian.html#link1
One missing piece regarding the idea of sustainability of ruminant farming versus grain farming is that the former tends to rejuvenate the soil, and can be performed in areas unsuitable for crop growth. The latter, on the other hand (unless performed in concert with ruminant farming), encourages deforestation (to locate healthy, nutrient dense soil), using up the nutrients in the soil, maintaining the soil via petroleum fertilizers, and then abandoning the land once depleted/eroded.
Ruminants are a key piece to the puzzle in sustaining the fertility of the soil when growing crops.
Exclusively eating local produce, grass-fed beef, free-range chickens and their eggs is not only seriously expensive, but in many parts of the US (like where I am) nearly impossible.
Whole grains and legumes, however, are pretty healthful and easy to grow, AFAIK.
Regarding economics, I suppose the unanswered question is how would a reallocation (or reduction, depending on your political bent) of government subsidies affect the feasibility/availability of grass-fed beef compared to the CAFO variety? What would happen if grass was subsidized instead of/in addition to grain?
Even now, it's possible to buy sustainable meat at comparable prices (http://www.getrichslowly.org/blog/2006/12/13/how-to-buy-a-si...) if you go in for a half or quarter cow. However, this involves quite a bit of work (if only it were as simple as logging into buymycow.com and ordering 200lbs to be vacuum sealed & delivered at your doorstep!).
You're making me hungry.