Hmm... Well, that seems to be the problem with GI_Joe. Its not true GI, and since it is using 64 skylights, it creates "artifacts". There isn't much you can do... Here is a pic of what I have come up with. I made all of the boxes and cyl polys, and extruded them in a bit. This pretty much stops the white lines that you get when you have two planes or polys tuching. for instance, check out the difference between the top and side of the ground plane boxes between the two images. If you add a point light to simulate sunlight, and if you ad a slight glow to the objects (even if it is really small), it helps "hide" the blochy artifacts. you can also just make the top have 256 lights. Tried it, takes forever to render, but Decreases the amount of Artifacts. I also tried making the ground lights have shadows, in certain cases it can really help or take away from a scene, I think that it helps with this scene. Well, let me know what you think, I can post the zip of the scene if you like, sorry if this ins't much help.
Brian