Peakguy has it right about balance -- plenty of good reasons not to load up on beef, but no need for us all to go vegetarian.

One aspect of mankind's biological and social advancement that we should preserve (even after peak oil) is our ability to see food as an art form and a cultural expression.  I think that means acknowledging the importance of animal products in these traditions.  It would be sad of too many people gave up gave one of life's greatest pleasures under the misplaced assumption that vegetables are magic health potions and animal products (even in moderation) are poisons.

Funny, my vision of post-peak oil was searching the countryside for wild berries and ant hills and stuffing myself with oats and other cereals. Maybe killing a (free range) rat or pigeon here and there for protein. Still, I agree that we should try to dress all that up into something presentable. :)