They usually use a "conservative date" to avoid the customer's wrath, but both the Best Buy and Verizon stores in my areas have already have received 2nd shipments.
As far as "gross underestimation of market demand" how can they know who wants to buy the Droid X, the Samsung Fascinate, the Droid 2, or the Droid Pro ahead of time?!? Their only true indicator are preorders.