Just my guess, but usually physics properties like mass, gravity, friction, restitution (bounciness), etc. can only be applied to physical entities that move. So (some) game entities should be able to have physics applied to them. Brush models (again game entities) could have physics properties applied to them as well.
With the materials type supported by the engine, it sounds like you could modify surface properties (like friction or restitution) based on the texture of the surface. So for example, wood surfaces would have a different friction than metal or glass surfaces), if that's what you are asking. You probably couldn't change the mass or gravity of something touching a surface, because that doesn't make sense in a real-world physics system.
Of course, all conjecture is purely bullshit until Valve actually releases the game and anyone designing a MOD around properties that can only be speculated about might find themselves in a deep pile of doo-doo when they discover that the engine doesn't really support the types of things that they thought that it would.
