They should have. They have people who's only job with the company is to analyze numbers and determine the amount of product needed for a roll-out.
Part of me thinks that they purposefully produced a limited quantity for the initial batch in order to have those magic words, 'sold out'.
Anyone who had access to a growth chart of iPhone adoption could've easily shown you that the demand for this roll-out would be exponentially higher than for the 3GS.
Don't get me wrong, I am happy for Apple's success, but make no mistake, they knew they would hit these figures.
Stories about systems going down and 600,000 sold are publicity. And you know what they say about publicity...