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The leaked source code for Diamond and Pearl shows that Platinum, as well as Gold and Silver remakes, were planned as early as late 2005. The file shown in this image, pokemon_dp/include/pm_version.h, has a last modified date of December 16th, 2005, 4:02 PM EST.
Platinum is referred to internally as VERSION_DP_NEXT, while Gold and Silver remakes are referred to as VERSION_GOLD and VERSION_SILVER. This same type of terminology is used to refer to FireRed and LeafGreen; they are referred to internally as VERSION_RED and VERSION_GREEN, as seen here and in their own source code leak.
VERSION_GOLD and VERSION_SILVER are also referred to in the file pokemon_dp/src/poketool/poke_memo.c, so that Pokemon from HGSS have a location of Johto when traded to DP.