But how will D/P know that Platinum is sending out a Giratina at all if Platinum is showing something belonging to a databank nonexistent in D/P? Like I said, Origin Giratina's databank will come after stuff like Egg, Manaphy Egg, Ground Wormadam, Steel Wormadam, etc. that Diamond/Pearl have after even Arceus' databank... Origin and Another Giratina's databanks are not the same thing, while in ADV Attack Deoxys and Defense Deoxys indeed had the same databank number in both games...
That is a fairly good point. I came to the conclusion that the alternate forms will have to be handled differently by using a more player-oriented approach:
Contrary to popular belief, Generation III did allow for Deoxys' true identity to be logically
deduced. Since Ruby and Sapphire only supported link battles, players could simply observe the opposing player's cartridge and infer Deoxys' form; the other player could of course do the same, or alternatively, pay attention to the trainer sprite. For wireless battles between FireRed, LeafGreen and Emerald, Emerald players had the luxury of being able to distinguish (based on trainer sprites) between Ruby/Sapphire, FireRed/LeafGreen and other Emerald players (but not between Ruby and Sapphire or between FireRed and LeafGreen). A FireRed player facing a Normal Forme Deoxys used by Red/Leaf could gather that the opposing player was using a LeafGreen cartridge, and vice-versa. Finally, a FireRed/LeafGreen player facing Brendan/May in wireless mode could know without a doubt that the challenger was using an Emerald cartridge, despite the fact that they appeared in their Ruby/Sapphire clothes.
In conclusion, with the aid of visual details and straightforward logic, only Emerald players facing FireRed/LeafGreen players were put in a disadvantage as a result of Deoxys' camouflage ability. In contrast, Diamond/Pearl players in either wireless or Wi-Fi mode will presumably not have a method of discerning between Land Forme and Sky Forme. Luckily, Giratina should be a different story due to the fact that Another Forme's Pressure activates a warning message, whereas Origin Forme's Levitate does not.
Of course, the point that you made also shows that displaying the Diamond/Pearl sprites for both Another Forme and Sky Forme is not as elementary as it was in Deoxys' case. In Generation IV, alternate forms are for all intents and purposes new Pokémon. Still, it is a stretch at this point to think that Platinum will not be battle-compatible with Diamond and Pearl. If nothing else, Platinum could rather easily have Origin Forme and Sky Forme shift to their previous forms when battling a Diamond/Pearl player; they would truly be Another Forme and Land Forme, respectively, rather than just appearing that way to the Diamond/Pearl player.
But having said that, the more important fact that should be taken into consideration is that Platinum will have new index numbers for Origin Forme, Sky Forme and the five silhouettes. Diamond and Pearl are the first games where there are practically no "pseudo-Pokémon": there are the 493 normal Pokémon, Deoxys' additional three forms, Wormadam's additional two forms, the Pokémon Egg and the Manaphy Egg. The Bad Egg is merely a glitched Pokémon Egg and has no data of its own. Ironically enough, Platinum will be the first game to add new Pokémon to an existing generation, in the sense that Origin/Sky Forme and Another/Land Forme will not overlap.
Game Freak could opt to take the easy way out and render Another Forme and Sky Forme unusable in battles against Diamond/Pearl players. The form-shifting mechanism is, after all, already at their disposal. One might think that the five silhouettes would be yet another bunch of alternate forms that could be treated in the same fashion. Personally, I find that that would not have an advantage over adding brand new Pokémon. They have already taken the step of creating new index numbers that do not exist in Diamond and Pearl, and if the new forms/Pokémon cannot be used in battles against Diamond/Pearl players, does it matter whether or not they can shift to different Pokémon? The moves would stay the same, but the stats and ability would not. Even if the new forms were somehow made usable in Platinum-Diamond/Pearl battles, that would only mean that so could new Pokémon. Unlike in the previous generation, new Pokémon and new forms are really one and the same.