I don't think they underestimated demand. I think they knew they couldn't fulfill demand but released it anyway, either to generate long lines, beat other tablets to market, and/or to fit their desired product cycle (e.g. move it away from the new iPhone launch).
Announcing a product just a week before release just doesn't feel like Apple. Maybe they over-reacted to the perceived lack of demand for the Verizon iPhone. Typically they would take some pre-orders or something. Doesn't make sense to give retail stores only a few hours between receiving the product and launch time.
A typical launch would have been to take pre-orders online for a week, then release either today or next week. This way they know what models are popular, can crank up production for those during the week, etc.