Today, Saturday, January 12, 2008 is Jun'ichi Masuda's (the composer of the music for pokemon and the main director of Diamond/Pearl) 40th birthday. On his blog he noted that people should play their copies of D/P on today.

While you might suddenly think that this is a legendary giveaway that is time Dependant. However, we are not this lucky. Sorry.

However, [Serbii.net is my source, and I'm quoteing directly here]

The community all jumped at this speculating on massive events expecting Darkrai, Shaymin and so forth to be given away. However, this was quite jumping the gun. Early reports are coming in saying that the suprise is the fabled Diamond Dust weather effect in Snowpoint City. It sometimes occurs on its own, but is quite rare. Like the snow that usually adorns the area, its essentially clear with small crystals coming down. It is purely a cosmetic change as theres nobody to battle outside in Snowpoint City. These reports are still unconfirmed but this is the only thing being reported from Japan regarding today's changes so far, and its unknown if it will translate across to US and European game cartridges. I'll update you as we learn more
Edit @ 22:40 GMT: These reports are now fully confirmed and we are receiving reports of this occuring in non-Japanese cartridges

I checked this today, and yes, its the diamond dust effect. Its gosh darn pritty and kinda cool. It does work on the american carts, so if you have acess to Fly and snowpoint. Check it out.