According to Digitimes, Foxconn has problem supplying all the units of iPhone 4 and iPad 2 that Apple orders. The production chain is running at its full capacity now and Foxconn is for now unable to create more units of production because of both a labor shortage and the lack of the necessary materials.
The shortage should last for the whole quarter. Also there are other problems such as the difficulty to obtain some components after the earthquake and tsunami in Japan.
According to some rumors and estimations, Apple would like to increase the production of iPad 2 up to 10-10.5 million per quarter, while it is currently limited at 7 million.