Most reports are indicating that the Wifi model is out of stock and the 3G model is on shelves in low demand.
Sony obviously should have made a second shipment of Wifi Vita's; the fact they are sold out and they didn't make a second shipment shows there is a shortage.
This is also why they only launched in Japan and not globally, they wanted to avoid the shortage they knew they were going to have (which still happened, just not for the 3G version).
The reason for the shortage is because the components the Vita uses are produced at factories that have other companies buying production time. There is only so much time and floor space in these factories, so Sony is only getting a small portion of the components they would need to produce the Vitas for a global launch.
Sony's only real mistake here was that they overestimated demand for the 3G Vita. The rest is simply external factors limiting production of Wifi models.
