My opinion is that the more consoles in people's homes the better. With such a small supply to each reigon at launch, they're pretty sure to sell out AT LAUNCH. But remember that Sony is not planning on supplying consumers the same way MS did. Sony will be shipping more units out over time for each of the original projected supply for US and JPN until the end of 2k6.
It was mentioned before that it was to prevent market conversion. Since this is done on the 20GB unit, and not the 60gb unit, its for price conscious consumer. If he/she doesn't get what he wants, then he/she might move to the other console for the time being. Sony plans on shipping more units over the course of the rest of 2k6, so the idea is to get the consumer's attention locked on PS3, especially at a time where it's high in demand, and user opinions are floating around the world at a rapid pace. Believe it or not, but conversion happens pretty easily and pretty quickly. Not a good move for shareholders, but a great move towards consumers that might have switched to 360 or Wii.