The DeleD lightmapper currently runs in a single thread. Functionality-wise, it makes sense to keep it a modal/blocking activity, as changing the scene would immediately render the lightmap invalid (as opposed to the raytracer), but making it multi-threaded would allow the lightmapper to use all cores in the machine. Currently, on a typical machine (dual core), when lightmapping a scene, you'll see one CPU be used 100%, and the other CPU be nearly unused. If you have more cores than just 2, things get worse :).