Tony Hawk's Proving Grounds was introduced at the Activision press conference and some details were given, then footage of gameplay was shown.
Proving Grounds concentrates more on defining you (your character) as a skater, by having you specialize in different types (character classes, anyone?) of skaters, such as "Rigger" skaters, who may, for example, take power tools to the "Skate-Stoppers" on benches and railings to allow some grinding action.
You also get to use the "Nail The Trick" and "Nail The Grab" modes to really get in and make your own impressive moves, helping to enhance the personalization of your character and your character's skating skillz.
In order to help build a sense of community, you get your own "Skater's Lounge," in which you can create your own skate-park with the included skate-park editor and then invite friends to check it out. Additionally, you can use an in-game video editor to create your own skating videos, complete with splicing and dicing and adding your own music. You can then upload these videos for sharing them with others and hopefully earn some money from your sponsors.
After talking with one of the developers at NeverSoft for a bit, they brought out a guest: Tony Hawk, himself, to discuss the game. He pointed out that you would even be able to do a sick move that has you landing on the board with it upside-down, sliding along on the end of the board. Too bad that wasn't in the earlier games; I usually land that way...