How can I unpack a .deb on Mac OS X without installing it?
ar is already installed on my mac on OS X. So just do
ar -x path/to/deb/file.deb
that's it. No need to install other software.
You can install dpkg using MacPorts or just download the sources.
if you use Darwin Port (now name it Mac Port) you can install dpkg and more package (like do it in debian or ubuntu).
after install Darwin Port go and install dpkg from here.
also you can find more info about dpkg in osx here.