First-class environments

This list has been pretty moribund for a while, so I'd like to introduce a new 
topic.  I've been getting interested in the notion of languages that support 
first-class environments i.e. environments as a data structure that can be 
grabbed and manipulated by a programmer.  The impetus for this was reading 
about metaprogramming in Ruby, which in that language amounts to being able to 
extend object environments at runtime.  This makes certain useful coding 
idioms possible e.g. reading from a database and creating objects that contain 
the database columns as fields.  This is used to good effect in web frameworks 
like Ruby on Rails, purportedly drastically reducing the amount of boilerplate 
code relative to similar frameworks in e.g. Java.

More generally, some authors have pointed out that many language constructs 
e.g. modules, classes, objects are to a large extent just variations on the 
environment concept, and functions are largely just code + environment.  So 
the appeal of first-class environments conceptually is that they might allow 
you to have a more minimal core language in which you could define whatever 
module/class/object/function semantics you wanted.

What I'd like to know are: 1) What are the best/most important/most famous 
papers on this subject?  2) What are peoples' opinions on this?  I'd 
especially like to hear about elegant coding idioms using first-class 
environments and also horror stories (if any).  For instance, are there 
certain important language features or optimizations that first-class 
environments make impossible?

Mike