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