http://www.serebii.net/diamondpearl/3rdgentrade.shtml
The Pokemon from the older games actually get transferred if you have Diamond and Pearl in the DS slot and either Pokemon Ruby, Sapphire, Emerald, FireRed or LeafGreen in the GBA slot. This isn't trading but outright transfer. Serebii.net seems unsure if the transfer is permanant, but if it isn't, then the obvious restrictions apply: transferring back only Pokemon and items that were available in the GBA games. Only 6 Pokemon per day can be traded, although I'm sure messing with the DS's firmware clock can remove that restriction. The features will probably be accessed after getting the National Dex in Emerald, or repairing Celio's machine in FireRed/LeafGreen. Ruby and Sapphire will probably work instantly. Likewise, something in Diamond and Pearl must be accomplished to read the GBA games.
Diamond and Pearl also connect with Pokemon Ranger, in order to capture Manaphy.
The Pokemon from the older games actually get transferred if you have Diamond and Pearl in the DS slot and either Pokemon Ruby, Sapphire, Emerald, FireRed or LeafGreen in the GBA slot. This isn't trading but outright transfer. Serebii.net seems unsure if the transfer is permanant, but if it isn't, then the obvious restrictions apply: transferring back only Pokemon and items that were available in the GBA games. Only 6 Pokemon per day can be traded, although I'm sure messing with the DS's firmware clock can remove that restriction. The features will probably be accessed after getting the National Dex in Emerald, or repairing Celio's machine in FireRed/LeafGreen. Ruby and Sapphire will probably work instantly. Likewise, something in Diamond and Pearl must be accomplished to read the GBA games.
Diamond and Pearl also connect with Pokemon Ranger, in order to capture Manaphy.