I don't know that it has been forbidden on the universal level for priests to use tablet-type technology in place of missals, but bishops have begun to frown on the idea and to discourage their own priests from doing so (example
). As the New Zealand bishops note in the story linked in the example, iPads and other computer tablets have a variety of uses, many of them profane (i.e., non-sacred). The Church's liturgical books are specially set apart for use during the liturgy. Whenever possible, use of ritual objects specially set aside for sacred use during the liturgy should be preferred to profane technological gadgets.