Due to a slip-up (presumably), it seems Videogames Blogger has managed to grab a screenshot of a premature announcement on the Pokémon Web site, regarding the release date of Pokémon Diamond & Pokémon Pearl in the U.S.
Nintendo announced this specific April 22 release date before they wanted to, because it was only online at Pokemon.com for a few hours (click on the Pokemon Diamond & Pearl logo in the middle of the page), before Nintendo took the new info off the website and replaced it back to the old spring 2007 vague release date. So you can’t see it there now, but trust us, like on the above screenshot we saw it with our own eyes on the official website.
In all likelihood, this probably was a premature announcement, and was, thus, taken down. However, there’s always that small chance that Game Freaks and Nintendo simply changed their minds about the release date and took the announcement down to reflect this. Keep your fingers crossed.