This morning my laptop stopped working, I switched to a mac mini, but hade only one monitor available.

The solution is rather simple (provided you have an X server running locally) :

ssh -X raspberry@192.168.0.42

it worked, but I could not sudo to start the python editor as root (which I needed to access /dev/spidev0.0 to play with my WS2801 led drivers).

One solution is (extracted from here):
Connect via ssh to the raspberry and :

apt-get install x11-xserver-utils
sudo touch /root/.Xauthority

Disconnect, reconnect and :

pi@raspberrypi ~ $ xauth list

This will show you some cookies values :

raspberrypi/unix:0 MIT-MAGIC-COOKIE-1 f0fdb32c00e54064bd5fe8969a6c227e
raspberrypi/unix:10 MIT-MAGIC-COOKIE-1 da868cd0f1f8b81b403562dc40f7bc1f

Then add the cookies to the root account :

sudo su
xauth add raspberrypi/unix:10 MIT-MAGIC-COOKIE-1 da868cd0f1f8b81b403562dc40f7bc1f

Wow, I can switch leds from a mac mini...