In answering a question about Mario Kart GP, the (currently) Japan-only arcade racer in development at Namco, Tom Harlin, PR Manager at Nintendo of America, has confirmed that Namco is also hard at work on another Mario game. In IGNcube's weekly "Nintendo Minute" feature, Harlin mentions that Namco is collaborating with Nintendo on Mario Baseball. No developer was announced with the project at its initial unveiling several months back, but Camelot was widely assumed to be involved, having developed all Mario sports titles since the N64 era.
Planet GameCube will have much more coverage on Mario Baseball later this month at the E3 Expo.
Update: One of our readers pointed out that this information was reported last November, also by IGN, although we're not sure where they got it from. The developer was not listed in game databases at Nintendo.com, PGC, or some of our major competitors...so even if this isn't exactly breaking news, apparently a lot of people didn't know about it!