The screen is my guess given that there is a company in Cupertino that effectively controls the touch panel market. I guess that's what happens when you place $5 billion worth of component orders at a time.
Clearly Asus knows something will turn soon as they expect the issue to be resolved by mid June. Shaking loose an extra 290,000 touch screens each month in just six weeks seems like a bit of a stretch as there are only so many manufacturers producing them and one company buying almost all of that capacity.
It seems that no matter the price other are willing to pay, Apple's ability to order billions of dollars in components months in advance rules the supply chain.