I've finally managed to do the first release of ssh-agent-applet. I've actually been using it for some time on my own PC but it needed a bit of love and documentation to make it easier to install.
From the README:
ssh-agent-applet is a Gnome applet that allows you to conveniently keep your ssh key(s) on external media. This means that if your computer is cracked or stolen, the attacker will not have a copy of private ssh key(s).
Using ssh-agent-applet, your keys are loaded into ssh-agent as soon as you insert your "key drive" into a USB port. The drive is automatically unmounted once the key loaded so you can remove it from the USB port immediately.
For developers, ssh-agent-applet is a reasonable example of how to write Gnome applets in Python and how to interface with HAL from Python using DBUS. The HAL/DBUS interactions are non-trivial and go beyond what most documentation covers. It took a fair amount of experimentation to get some of this stuff working. Hopefully the applet or at least its' code is of use to other people.
Downloads can be found on my Code page.