I'd been looking for this reference for a while, and it dropped right in my email inbox this morning from a colleague:
Computing is the only profession in which a single mind is obliged to span the intellectual distance from a bit to a few hundred megabytes, a ratio of 109, or nine orders of magnitude. The immensity of this ratio is staggering. As Edsger Dijkstra says, “Compared to that number of semantic levels, the average mathematical theory is almost flat. By evoking the need for deep conceptual hierarchies, the automatic computer confronts us with a radically new intellectual challenge that has no precedent in our history” (“On the Cruelty of Really Teaching Computer Science” Communications of the ACM, December 1989).