A column at Mainichi Digital claims that PlayStation Vita's initial shipment may exceed 700,000 units.
The column's writer, Fumio Kurokawa, says that he's heard from various sources related to the matter that Sony originally planned an initial shipment of 500,000 units. However, due to pre-order demand, Sony added an additional 200,000 units.
Many retailers, including Tsutaya, Yodobashi Camera and Bic Camera, have said that you can purchase a system on launch day without a pre-order. A sudden 200,000 unit boost would explain the availability of stock.