Blizzard wanted to keep it a secret for an announcement at the next Blizzcon, but their business partner spilled the bean: Nintendo leaked the news deeply hidden in their quarterly report that they sold the license for a "Pokemon MMO" to Blizzard. That fits with Blizzard's earlier statements that their next MMO would not be a WoW2 or be based on any other of their existing brands.
The writing was on the wall, with World of Warcraft becoming increasingly casual-friendly, so we shouldn't really be surprised that Blizzard's next MMO is going for an even more casual (and thus larger) audience than WoW. They probably realized that the competitor's game most likely to be a huge financial success this year is Free Realms, and not any of the more classic MMORPGs. And combining the formula of a kid-friendly virtual world with the addiction of leveling up your trainer and your pokemons could well become a game selling millions. Added advantage: The Pokemon MMO wouldn't canibalize WoW's user base.
I played some of the Pokemon games on Nintendo handheld consoles, and they aren't half bad. I really like the separation of having one trainer and many pokemons. So things like reputation are centralized, and you only have to grind them once per account, but you can easily switch to a different "class" / pokemon for experiencing different roles in combat. Add a nice virtual 3D world, and Blizzard's usual polish, and this could be a game I want to play, in spite of not being a kid any more.