Well we've had Mario Golf, most recently Mario 'Power' Tennis, so handing sporty spice Mario a baseball is not too much of a stretch.
Nintendo is planning to release Mario Baseball somewhere in 2005, but apart from that there are not many details available at all.
The screens do all the talking for now, as apart from Mario himself, we can make out the usual characters such as Yoshi, Luigi, Bowser, Wario, Peach Koopa, and Waluigi. There also seems to be some sort of power swing similar to what we have seen in Mario Golf and Mario Power Tennis.
Mario Baseball is being developed by Namco, no stranger to the sport as they make Family Stadium Baseball in Japan, known in the U.S. as RBI Baseball.