It actually was originally written in Lisp, and AutoLisp is still used to interface with it. (This is how I started learning Lisp in the first place, to create 3d models of designs with a little Lisp help.)

Here's a quote from related email discussion (Jan. 2009):

"Here's an indisputably real-world example: AutoCAD. Most of it is written in a customized LISP; the C part is the LISP interpreter core." -- ERS