Well, yes and no. I should have been clearer in my original response, but PRT lighting IS a global lighting technique, but thats not the same thing as 'Global Illumination', although they achieve very similar results. Confused yet?

PRT lighting does emit from a singular source, and uses the radience reflected off of the environment to light objects and characters within it.
But you are also correct when you ask about multiple lighting systems. Both PRT and global illumination are better at replicating skylight (the cool toned light relfected off the atmosphere), but leave room for desire when it comes to hard sunlight. Usually the most realistic lighting is achieved with a global light combined with a strong directional light to cast hard shadows. I'm not completely sure how the lighting is being approached in Halo3, but generally only one global light emmiter is used per scene. When you have an indoor scene with multiple light sources, then traditional lighting techniques would still be the best option. However, it sounds as if they aren't going to use object specific light linking anymore... so technically the lighting should still be unified regardless of the scene or techniques used.
very cool......