Pokemon Company and Tecmo Koei are teaming up for a crossover involving the Pokemon and Nobunaga's Ambition (Nobunaga no Yabou) franchises. The new game, co-produced by Pokemon Co. and Tecmo Koei, is Pokemon + Nobunaga's Ambition and is due for DS release in Spring 2012.
This is the surprise Pokemon game that Pokemon Company was teasing for its its first showing at the Jump Festa event. The walls of the Pokemon booth were lined with artwork and screenshots, but no actual details on the game, aside from it being a strategy title, were revealed.
Based off the images, it looks like the game pairs Pokemon creatures with historical generals.
Visit 4Gamer for pics from the booth and more screenshots.