Although I no longer use packagekit, I still have my apturl script so I thought I would post it. This script will allow you to open apt:// scripts with packagekit. (Just save this to a file, mark it executable, and tell your browser to open apt scripts with it).

#!/bin/bash
/usr/bin/gpk-install-package-name $(echo $* | sed -e 's/apt:\/\?\/\?//')