I started up Pokemon SoulSilver, chose the Mystery Gift option, selected Receive Gift , and specified Get Via Nintendo WFC.
After a few seconds of checking, the screen came back and said it was downloading Make a Wish Jirachi.
It looks to be the similar to the Jirachi they were giving out in the GameStop download event back in March. It is level 5 and holding the same item, a Liechi Berry. The original trainer (OT) is listed as SMR2010, no doubt for Summer 2010. It knows the moves Wish, Confusion, Rest, and Draco Meteor.
Having a Jirachi is Pokemon HeartGold or SoulSilver opens up a new Pokewalker route.
I expect that Nintendo will announce this as their current WiFi download at some point soon. Right now, however, the site is focused on the Pokemon Video Game US National Championship, which took place this past weekend.
This download is only for Pokemon HeartGold and SoulSilver. It does not appear to work with Diamond, Pearl, or Platinum. Instructions for setting up Nintendo WiFi on your DS are in the instruction booklet that came with your game.
More on this when Pokemon makes an announcement.