It's probably mostly for fixing the glitch that allows you to get Shaymin and Darkrai without getting the promotional items for them. Of course, no one is going to want to do that, so Nintendo will have to include some sort of incentive for doing this. If that theory is right, then it's possible that this will be a Japan-only thing, since that's the only country that got the Darkrai/Shaymin glitch in the first place. Just a theory though.
Edit:
Wow nevermind, I had no idea about Platinum. If the rumors are true about their being alternate forms of Giratina and Shaymin in it, then it would make sense that the patch is to prep Diamond and Pearl to be able to have these new forms traded over. On the other hand, the same thing could be done whenever a Diamond or Pearl version connects to Platinum for the first time. Hmm, well I guess that's just another theory.