From the press release:
Jon Kleinberg's work has brought theoretical insights to bear on important practical questions that have become central to understanding and managing our increasingly networked world. He has worked in a wide range of areas, from network analysis and routing, to data mining, to comparative genomics and protein structure analysis. In addition to making fundamental contributions to research, Kleinberg has thought deeply about the impact of technology, in social, economic, and political spheres.
The Fields Medals go to Andrei Okounkov, Grigori Perelman, Terence Tao and Wendelin Werner. Perelman as expected for his work on the Poincaré Conjecture. Terence Tao played a role in work on Gowers Uniformity as well as many other areas of mathematics.
Finally Kiyoshi Ito wins the first Gauss Prize for Applications of Mathematics.
Luca is in Madrid and has the on-site ICM perspective. According to Luca, Perelman has declined the Fields Medal.
Update: BBC Story