nmw:I wasn't talking about programming in Emacs Lisp, I was talking about using Emacs to write Common Lisp code and interact with a Common Lisp system. ilisp rocks.

Extended conses aren't going to happen. A two word data structure that's basic to the language would be transformed into a multiword structure... besides, compilers and codewalkers and such quickly transform the list-based source into a structure-based representation. If you really want properties associated with cons cells, you can do it with a hash table.