we've developed a piece of hardware for controlling large arrays of addressable LEDs, and a library to connect it to Processing.   PixelPusher implements an arbitrarily scalable, self-enumerating, high performance network interface for addressable LEDs (both the flexible LED strips and the LED pixels.)

The current version of the library is available here (it's open-source, with the source being here.)

We have some documentation available here that describes how to use the library and how it's supposed to work,  and you can get the hardware (which might be the first hardware specifically designed for Processing?) here.