There's a pretty good demo in the new Official Playstation Magazine, which includes samples of nationals, motocross, hill climb, and machine race. Nationals and motocross havent changed from ATV 2 and MX Unleashed, just new tracks (though it would be cool to see some of the better tracks from previous games return). The machine race is a wide mellow track designed for the new sand rails, which handle like a cross between the buggy and trophy truck of MX Unleashed; pretty fun and easy to drive with the new cockpit view. The hill climb is pretty solid, and is more fun than just running up the bordering walls of a normal level, because of the improved mechanics and camera angle. Also notable in the demo is the option to have hand guards, so there might be other little aesthetic options in the final game. Other additions to the series that arent in the demo include short track races, gap jumps, online (new to MX series) and supermoto.
The game should be pretty good, because its made by Rainbow Studios. They made ATV Offroad Fury 1 and 2, but not 3--which is unofficially ATV Quad Power Racing 3, which was cancelled when Acclaim went under, but bought up by SCEA and published under the Offroad Fury guise. Rainbows other titles include Motocross Madness 1, 2, and MX Unleashed, which is part of the series by THQ that includes MX Superfly, and MX 2002 Featuring Ricky Carmichael (both by Pacific Coast Power & Light).