Havak
I'm the Best. You're a Towel.
According to filb.de, the 'Enimga Stone' Event Item will be available for download via Mystery Gift and Nintendo Wi-Fi Connection from July 31st - August 27th. This is the European Event most likely, as Germany is the first place to have it announced. However, since every European Wi-Fi Event thus far as been accessible in America and Australia as well, it's pretty safe to assume this is more of a worldwide Wi-Fi Event.
For those of you who don't know, if you take this item to the Museum of Science in Pewter City, Steven Stone will be there and have the item examined. You'll discover that it's actually the Soul Dew item. Upon leaving the building, the currently unobtainable Lati-twin from your game version will attack you and you'll be able to capture it. If you defeat it or run from it, it'll reappear so you should have a decent chance to get a good one.
Obviously, you can only download this on HeartGold and SoulSilver.