You can use an Ipad once the seat belt signs have been switched off, until they are switched on again for landing.
SNS3Guppy has supplied a very good reason that you can't during the "critical" flight phases, however, there is another reason.
If you're engrossed in your scrabble or angry birds or other such Ipad pass-time, then you're not concentrating on what's going on around you. It's more than likely then that you'll miss an important PA or instruction in the event of a problem
.
This is another reason that you can't use it willy nilly, the pilot's will be using theirs for a specific purpose, not amusement.