The company says it'll continue to monitor demand for the 20GB version, of course, and claims that it's only going with the 60GB version as the sole offering because that's what retail has told them to do.
Whether that's entirely true or not (we've spoken to several retailers who seem bitterly disappointed at the decision to launch a single SKU and not offer a lower priced proposition), it's obvious where the decision has its basis.
Sony are no doubt mindful of the fiasco at the launch of the Xbox 360, when Microsoft chose to ship a large number of crippled Core systems as part of it day one allocation - leaving frustrated consumers who wanted the premium system stuck with buying a Core, and then battling over short stocks of the hard drive add-on.