Rotom has more forms than any other non-legendary Pokemon, and when you battle it in the generation IV games it's the only non-legendary Pokemon that uses a legendary Pokemon encounter theme.

In the non-English European versions of Pokémon Diamond and Pearl, Registeel's sprite was changed because the position of its left arm was similar to the Nazi salute. In Pokémon Platinum, the sprite was changed for all regions to the continental European variation. This change was retained in HeartGold and SoulSilver.

The title screen music for Pokemon Ruby, Saphire, and Emerald can be found in the code for Diamond and Pearl. It is labeled "MIDI HEX 0003" and was likely used for testing purposes during development.

There are 9 unobtainable items within Pokemon Diamond & Pearl. Some were removed entirely, some were replaced with updated versions, and some are only available under certain conditions.

These items and descriptions are as follows:
Park Ball - Captures Pal Park-transfered Pokémon without fail (obtainable in game but only in Pal Park)
Cherish Ball - Normal chance of catching a Pokémon (Pokemon received from official distributions come in this ball)
Boat Ticket - Ticket for the Luxury Liner SS Anne
Contest Pass - Allows entry into Pokémon Contests
Magma Rock - Mystical item that maintains the Stark Mountain's volcanic activity
Red Chain - A mythical item created by the Sinnoh Legendary Trio to awaken Dialga/Palkia
Rule Book - User's guide to online trading
Seal Sack - Holds up to 10 Seals
Treasure Sack - Holds treasures found Underground

Within Route 218, inside one of the hallway type buildings, there is a guard who will tell you how thirsty he is. However, bringing him a drink of any kind won't do anything.

This is a reference to the original games, Red/Blue/Green/Yellow, in which the player must bring the guards a drink to enter Saffron City.

On January 12th (Director Junichi Masuda of GameFreak's birthday), Fly up to Snowpoint City and instead of the usual flurry snow storm, there will be glittery diamond snow. There are also a number of other dates that this Diamond Dust can be seen.