As Darwin is the bottom-most layer of MacOS X, any Common Lisp implementation that works there will work with Darwin, with the exception of any that require Apple's proprietary Aqua GUI, of which there are none. LispWorks, for instance, can use Aqua, but will work without it, from the command line.
Darwin even works on the Intel x86 (PC)!
Darwin can be downloaded from opensource.apple.com.