There's a bad guy that is jealous of the Hulk's awesome power, so he becomes an equally enormous monster. The two dual and The Hulk wins. While that's not a surprising ending to a comic book movie, its even less eventful when The Hulk is portrayed throughout the movie as completely invincible. It removes any suspense of danger.
More exciting is that Samuel Stern is introduced. His character is one of the more interesting in the movie and will definitely be something to look forward to.
Finally, I will save everyone 5 minutes of their lives and tell you not to stay until the end of the credits because there is nothing there. As you probably know, Marvel Studios is producing a series of movies, that will all lead to what will no doubt be the most anticipated movie in decades: The Avengers in 2011. The Incredible Hulk teases this upcoming movie by adding a scene at the end that is so distracting and out of place it almost ruins the entire film. Instead of leaving the theater absorbed in the story of The Hulk (which might have happened if they cut to the credits after his eyes turned green), the audience is reminded that Iron Man was a far superior film.