Related to MarkM's answer (a CDR disk is a small explosive device):
A few years back the R/W speed of CDROM drives was increasing faster than the makers of blank CDs could manage, and CDR disks were exploding under the strain of being made to spin above their design speeds.
So ... distribute software on really cheap CDRs and somehow contrive to spin up the disk until it explodes.
EDIT: OK, let's avoid damaging the drive.
How about a chemical which doesn't corrode the disk surface (as detj suggests) but a photosensitive coating which becomes opaque after exposure to the laser used to read it the first time?