This is backwards-incompatible, because this variable was public and documented (see http://nltk.org/api/nltk.html?highlight=nltk.data#nltk.data.path), and people seems to use it, e.g.: http://dragonlist.wordpress.com/2011/04/30/nltk-add-new-corpus/, http://stackoverflow.com/questions/3522372/how-to-config-nltk-data-directory-from-code.

Singular "path" is common, e.g. Python's sys.path or PATH environment variable. I think we shouldn't change this name.