In theory, if you say that supply is going to be tight, more people are likely to rush out and buy it "while they have the chance." You create a rush, as people will be afraid to miss out. However, there is no shortage, and all of the people rushing to get the console will be able to buy one, as will any stragglers, and anyone else who wants one. It is a psychological device to trigger the purchase impulse in people who are pensive, or trying to hold out for some reason. It is sort of like a kid with a toy - he may not play with it, but when he finds out someone else has it, and he doesn't, he suddenly wants it very badly.
Of course, I don't know if that is the case. Tactics like that are a gamble, as sometimes people are dissuaded, and figure that they might as well wait it out, rather than trying to deal with lines, and such. Given the success of the system thus far, and potentially key software releases in the near future, I am hesitant to say that Nintendo is, in fact, manufacturing shortages.