If you're interested in developing OS X applications with a lisp, you should check out Nu — http://programming.nu — It's a Cocoa-native language built on the Objective-C runtime, that's flavored somewhat like Common Lisp.
I've developed stuff on Nu before, it's magically easy compared to other language binding frameworks for Cocoa. I still use nutest as a testing infrastructure for projects that use exclusive objective-c.
There is an interresting development (but haven't checked it lately) for using Cocotron (Cocoa clone) for Windows for ClozureCL.
What's really cool, is that while Objective-C Win32 applications written with Cocotron must be compiled under OSX (using gcc-mingw tools), this allows you to write them under Win32, because it uses the produced DLLS (AppKit.dll, Foundation.dll) and you communicate with lisp with them.
There is one thing to note - ClozureCL has 32-bit and 64-bit executables, but for some reason the 32-bit produced ones do not work on 64-bit.
That might be fixed already, I have to check again.
Additional info:
● A talk by the gentleman who created Nu, Tim Burks, talking about why he did so: http://www.viddler.com/explore/rentzsch/videos/13
● The Google Group for Nu: http://groups.google.com/group/programming-nu
● The source (Apache License) on Github: https://github.com/timburks/nu