I don't see what is wrong with using an iPad for celebrating Mass. Although I think that the Missal as a book should be the norm, but I see how this technology can be useful for Priests.
On Saturday 7 July 2007 Pope Benedict XVI issued an Apostolic Letter on the celebration of the Roman Rite according to the Missal of 1962. The following text is the unofficial Vatican Information Service translation of the official Latin text.