Hey, once again, this might have been brought up in the thread created earlier about this topic...
From IGN.com
Having Havok?
Rumor: Super Smash Bros. Brawl will utilize the Havok physics engine.
This rumor has been floating around for a while, but recently found new legs thanks to a bit of HTML wizardry on the official Havok site. Magically, an entry listing Brawl as an official supporter of Havok appeared, then just as quickly disappeared. The fleeting confirmation is really all this rumor needs to be accepted as fact, but as the entry's been taken down it's not officially solidified as yet.
Havok is a bit of middleware software that developers can use to make characters and objects in their games react to each other more realistically. There have been several recent games to take advantage of it, including the physics-gone-mad epic Half Life 2 and the recent PS2 Mana series installment, Dawn of Mana. In those games, the more realistic reactions of enemies and objects allowed for Gordon Freeman to have fun moving oil drums around with his gravity gun, and Mana warriors could laugh when a Rabite flew back into another Rabite and both rebounded away from the resulting collision.
If Havok is indeed in Brawl, it would be a dynamic change to how battles played out – characters would bang into each other in mid-flight, or you could throw one of your enemies at another to take them both out. Items and elements of the environment could be more interactive as well, and the fact that crates and barrels have been revealed already to slide and roll down inclined slopes could lend further credibility to this rumor.