Bought a giant LoL (lots of LEDs) shield from Olimex on Ebay. This shield has a matrix of 9x14 LEDs which can be individually driven. The shield has pinouts for Arduino boards, but wiring up 12 digital outputs from the ATmega chip on the Gertboard (PD0-PB5) to 12 digital inputs on the shield (2-13) allows the arduino IDE on the pi to drive the shield perfectly. Take a look at arduino images on the web to see where the digital pins 2-13 would connect to your shield.
Download the library from http://code.google.com/p/lolshield/ and place it in ~/sketchbook/libraries (create these directories if they don't exist), then restart arduino IDE if open, and finally open the LolShield/examples directory in your arduino IDE and upload.
Note that I had to placate the compiler with a couple of 'const' references in the LolShield example/library code, and I had to remove the programming wires from the Gertboard after uploading to prevent the pi's GPIO pins from interfering with the ATmega chip at runtime.
The LoL Shield library lets you do loads of cool things from scrolling text, to vu-meters. I'll be having fun for some time, I think!