`Declarative` is a good suggestion.  Another idea: `RSpec::Core::MemoizedHelpers` -- since both `let` and `subject` define memoized helper methods.

> This may be getting a bit ahead of myself, but do you have any policy regarding deprecating constants (modules)?

We try to follow SemVer but there are gray areas for cases like these.  In a case like an error class that I expect someone to explicitly reference (e.g. to rescue that kind of error) I'd definitely want to properly deprecate it using a [technique I blogged about](http://myronmars.to/n/dev-blog/2011/09/deprecating-constants-and-classes-in-ruby).  That said, I don't consider the constant names for the `Let` and `Subject` modules to be part of the public API; it's simply the internal code organization we've chosen to use within rspec-core.  The public API are the `let` and `subject` methods made available in example groups for end users, and the modules they come from shouldn't matter.  I can't think of a case where a user would be explicitly referencing one of these modules.