Wow. Lots of strong feelings here on this thread!
No-one has mentioned bio-accumulation yet, tho. Remember DDT? It was banned because it accumulates in living tissue. This means that plants pick it up and can't get rid of it. An animal that eats plants gets the DDT and can't get rid of it. Going up the food chain, the DDT is more and more concentrated until it causes problems (such as the American eagle had, the birds could not lay many viable eggs). I wish I could provide links, but I am a book lover and have my copy of 'Silent Spring' (by Rachel Carson) right here.
It is probably possible to use DDT in a responsible manner, rather than spraying large areas of the world with it. However, many chemicals are difficult for living things to get rid of. Many of them come from various forms of waste, like dioxin. Others have uses, like the large PCB family of chemicals, but escape into the environment. Very little research is done on the effects of these chems on living things, probably because they are not intended for use with living things. And the MSM covers this not at all, AFAIK.
Humans living in areas where crops are limited or impossible (such as the Inuit, or eskimo) very often have high concentrations of these chemicals in their bodies. Most of the time, there doesn't seem to be many problems, even if there are more susceptable animals out there. However, I don't see the world getting cleaner. I don't think there will be much extra cash around for making sure waste dumps don't leak or that accidents get cleaned up. You pays your money, you takes your chances.
That's my plug for vegan living today.

BTW, if you go vegan for a very long time and then eat some meat, you will prolly get so sick in an hour or two that the temptation to eat a well-made hamburger at 2 am will be easily resisted no matter how hungry you are.