You'll be able to download your own face textures and put them directly onto your skater's head. It's currently unknown if those textures come from the Internet or from an Eye Toy-like camera device. You'll also be able to create and name your own trick animations. The interface for this appears to use keyframing, and it looks a bit like the animation creation found in games like Fighter Maker.
In the "new gameplay feature" department, you'll be able to actually get off your skateboard. This means you can get running starts, jump, and hop on your board. You can also climb ladders and hang from ledges and wires. It is also believed that players will be able to interact with vehicles.
The create-a-park mode has been expanded quite a bit. You'll be able to create goals of your own, and it appears that you'll have a much larger area to work with. You'll also be able to place full-size buildings in your levels
The game will be released on ps2, xbox and gamecube. Most likely it will be eventually released on pc.
The trailer for tony hawk underground can be downloaded here