I've wondered about this too. When I was researching MacBooks before taking the plunge and buying one, I read some complaints where MacBooks were scratching disks. I watched a DVD with the MB on my lap, and checked the disk afterwards. No scratches. The other day when I was ripping a DVD to put on my iPod I absent-mindedly picked up the MB and set it down out of my way, realized it later, and checked the disk. Once again, no scratches.
This lead me to believe that perhaps this was a problem with the earlier MacBooks, because I have yet to scratch a disk, let alone hear anything else about the scratching problem.