A student of mine (Andreas Kirsch) just recently finished his implementation of light propagation volumes. His implementation is available under the very liberal BSD license, so be sure to grab it while it's hot. There's also a bunch of corrections in there, so it might be also interesting if you've just read the paper so far. The implementation is based on the updated I3D 2010 paper, and not directly on the 2009 SIGGRAPH paper. Of course, feel free to ask in the comments here or at Andreas' page if you have any questions.