In general, how much weight will the typical employer (i.e., law firm, government department, etc.) give to a special certificate obtained with an LLM? In other words, is a lawyer interested in environmental law better off obtaining a LLM with a special certificate in environmental law from a law school ranked say 22nd, or an LLM from a law school ranked say 12th that does not offer a specialization in environmental law? Does anyone have a general sense of how this goes?

I would think the special certificate would help you find work in that specialized field, e.g. environmental law (in the case of Stanford and Berkeley), or business law (in the case of Wharton/Penn Law and Kellogg/Northwestern Law). However, even with a general LLM (without any special certificate) you may still be seriously considered for that same type of work, depending on your work experience or on how well the interview goes. If you can enter a prestigious LLM program that would offer a special certificate, it would afford you the best of both worlds.

Employers tend to give weightage to the best programmes in their field of concern. They would not be naive not to know what courses are offered where. Thus, ignoring a better ranked university in favour of a lesser ranked but specialist university, would only show your focus and determination and leave you in good stead.

One has to remember that rankings are based on generalised inputs and there are no rankings for specialisation within law. Thus, it would be safe to ignore rankings in case one is looking at an institution that has great reputation in the field concerned