The optical limits of semiconductor fabrication I discussed in my previous article (http://www.overclockers.com/articles1170/) apply equally to optical media. Unlike semiconductors, there are some workarounds - namely, holography.
Standard optical media stores information as pits in the disc’s data layer. A laser then reads these pits. The size (and density) of the pits is limited for the same reasons as semiconductors. You can think of it as Morse code - the shorter the blips, then the more information you can pack into a given length of time.