I am using Unity Pro 4.1. I had been working on a fairly large environment, and had been using the Beast Lightmapping already. With the given resolution and other values, the scene had been taking about a half hour to calculate all of the lightmaps. I needed to make alterations to one of the models ( a large elevated walkway), and replaced the model that was there with a new one. This model and all others had already been set to calculate lightmap uvs.
For some reason, after I put in the new model and attempted to make the lightmaps again, it went so slow, that after several hours I assumed something had frozen, because the progress bar (green bar visible from the unity icon in the task bar)had not moven at all. I have been letting it run now, for over 8 hours, and it has been slowly progressing (estimating by the progress bar it has gone a little over 5 percent. The progress bar in the lower right hand corner of the Unity window says it has progressed to the GI Calculation stage (pass 1 of 5). Its not dead, but it is taking so long, that it is completely unrealistically usable.