The game is titled Pokemon Zircon (Jirukon in Japanese, which translates to Zircon), and although all the details are not yet known, some information about the game has been released:
Unlike previous third versions, which left the plot of the game primarily the same, Zircon will feature several large changes to the plot and adventure to enhance the story. Giratina will play a much larger role in Zircon, and you will be able to encounter it before fighting the Elite 4. (How this will work exactly is unknown, because it isn't on the Sinnoh Dex... maybe you won't be able to catch it? maybe it will just have "???" for its number?) There is a new area to the Northeast Island called the "Challenge Area," which will contain simulation fights similar to Pokemon XD, as well as some competitive-style battles with twists thrown in. (Sort of like the Battle Frontier from Emerald, I'm guessing.) New applications for your Poketch!
Information is very limited at this point, although I am sure we'll be hearing more about Pokemon Zircon in the upcoming months before its release date, which is scheduled for Fall '07 in Japan.
Yes, as I'm sure many of you figured out, the box art for Pokemon Zircon was a fake. I tried to make it as believable as possible, and I've gotta say I don't think I did half bad.
Although I think some of the features I mentioned would be neat, no third version to the D/P series has been announced, not even in Japan. Throughout this whole thing, it really did make me wonder how the third version (if they do make one) Legendary will work, since the only other Legendary in the Sinnoh Dex is Manaphy, and you get that in a different way. Curious...
I apologize if I upset or mislead anyone, and I hope you enjoyed this year's April Fool's Day surprise! It's all in the spirit of fun...
Also, a quick REAL update: a bug in the size comparison feature of the Pokedex has been fixed. Now, smaller Pokemon should be displayed at their proper height rather than look reaaaallly tiny.
*All content are adapted from Pokemon Marriland