Yeah, that guy is nuts. I fold on my gaming rig when not gaming. I might as well help out medical research with my poweful GPUs, who knows, maybe we will cure everything. The power of distributed computing is really staggering.
Yes, because you could run a computer (well, cluster) with 23 GPUs (in this case, 2 GPUs/card, so 46 actual GPUs), but you wouldn't see much/any gain after 4 GPUs because SLI and Crossfire are limited to 4GPUs. If your application can take advantage of CUDA (Nvidia) or ATI's version (I forget -.-), you can see HUGE gains, like with 3D Modeling. I've got a friend who has 10-12 9800GX2s that he uses exclusively for 3DS Max, Maya, or Blender (I forget which).
No, because...well...like I said above, you could only use 4 GPUs max if you're only gaming.