Some time ago, Han Feng of Hyperbody [hyperbody.nl / makeahybrid.org] did a really great job and modified the old 3d voronoi grasshopper definition and made it much more sleek and streamlined. In all good faith, here it is: http://improved.ro/wp/wp-content/uploads/improved//2011/06/110602-3D_voronoi.zip.
To make it work, you need to download qhull. Get it here: http://qhull.org/. After you install it, double click the scripting component and edit the path to your qhull folder (line 101). If things do not work, take care to unblock (right click and fiddle with the permissions) the qhull execs (sometimes window$ blocks them).