This isnt really a cheat but instead of catching all of the pokemon to get the national dex, in diamond and pearl all you have to do is SEE all of them. If you beat the champion and fight most or all of the trainers, you should see all of the pokemon. If you are playing diamond, you can see a picture of palkia in a big hous in celestic town which will count as seeing it. Same thing with pearl but you will see dialga.HOPE THIS HELPS!