The lamp which I used was a "USB Glitter Light" available at low cost from your favourite online auction site, local market or pound store.
Therefore I designed a simple circuit around an FTDI MM232R module and some MOSFET drivers to accomplish this, also adding a piezo-beeper to alert developers when a build fails. The prototype was simply built on old-fashioned stripboard (0.1" pitch).
I used a self-drive piezo-electric beeper to provide an audible alert.
All of the documentation is contained within the Hudson plugin LavaLampNotifier.hpi as LavaLampElectronics.zip. You will need to use jar or unzip to extract it.
Since the hudson server is back in the server room, the lava lamp module is attached to USB on a desktop machine and a simple socket server runs to listen for build results.
FTDI provide free USB drivers for a number of operating systems. Higher level code still has to interface with the driver - a SourceForge project, FTD2XXj, has already done this, so writing a Java program to control it becomes very straightforward.
The lamp control software is also zipped up inside the released Hudson plugin as LavaLampController.zip. Extract it from LavaLampNotifier.hpi , then unpack it to a directory on your Windows or Linux machine. An example Windows batch file lavalamp.cmd is provided which should get you up and running.
The notifier plug-in talks to this server across the network. (I could have used an existing RSS notifier, but didn't want the lamp to be polling Hudson constantly, I rather preferred the immediacy of the result being "pushed" to a known listener.) It consists of a "global" module which allows configuration of any number of different LavaLamp host/port combinations (to support different jobs or projects).
A "per-job" module then allows selection of the lamp you want to notify with the status of each job.
Theoretically alternate "lamp server" modules could be developed which might not necessarily use a hardware lamp at all - listening on an IP-multicast address to pop up an icon in everybody's windows tray is one idea.