Running juju locally on 12.04 (or getting over agent-status pending)

I've just tried following the instructions here to get juju running locally on my brand new Ubuntu 12.04 (desktop) system.
No matter what I tried my wordpress and mysql charms seemed stuck in agent-status pending and ip address null. Even waiting a couple of hours.

I needed to take to a couple of extra steps to get it working that didn't seem to be documented in one place. So I thought I'd put them both here.

All it took was to add a rule to ufw:

sudo ufw allow from 192.168.122.0/24 to any

Hope this helps someone who was stuck in the same way I was.