IGN has posted an article covering NaturalMotion's Euphoria Technology and Grand Theft Auto IV. Here is a quick list of some of the amazing things Rockstar have implemented it in:

• Niko's stance will adjust depending on the grade of ground he is standing on
• Niko is also spatially aware of his environment (in layman's terms, he knows where walls and potential dangers are)
• While standing on hills and mountains his knee's and feet will incline or decline on an angle
• He walks with a noticeable counterbalance, keeping his center of gravity correct
• When running at a fast speed he will be proactive in pushing open doors (because he is spatially aware of his surroundings, he will know the door is there and thus push it open without stopping)
• When shot enough, enemies will fall backwards and slouch, trying to regain their last ounces of breath.
• Car physics are sickenly realistic; rear-end another car and the people in the front seat will propel through the windscreen.
• Same goes for side-on's, bailing from the car while in acceleration (similar to what you could do in Vice CIty onwards, only a little more realistic now)
• When tackling walls, staircases or even just falling Niko's hands and feet are controlled automatically to ensure all his movements are realistic and play true
• Niko will duck into a commando roll when he falls to far
• Euphoria also controls how the new drunk feature works -- drunken, stumbling shamble, and camera blurs adding to the disorientation
• When clinging to trucks (as seen in trailer #2) he will hold on more forcefully when the truck takes corners
• Pedestrians are also proactive to weather formation and will change their actions (for example running) when it is going to begin to rain momentarily

Very impressive stuff, we're just itching to play this game just to see how the physics work. There is no doubt, this game is going to re-define gaming a second time for the franchise. Be it the game play that is being raved about of the physics that have never been attempted before, this is unmissable.

Adapted from IGN, thanks guys!