The International Congress of Mathematicians is held every four years, and the next iteration, ICM 2010, will be held about a year from now, in Hyderabad, India. These are huge conferences, planned well in advance, with 1465 mathematicians already pre-registered.
The list of speakers gives a good indication of what the mathematical establishment views as the most important research activity of the past four years, and this list is now available here. There are a large number of parallel sessions, and a limited number (20) of plenary talks.
The winners of the Fields medal are announced at the ICM, at the same time as the composition of the committee that made the choice (the chair of the committee, Laszlo Lovasz, is known). One way to help guess who will win a Fields medal is to take a look at those on the speakers list who are under forty. I’m not privy to any inside information, but many people think Ngo is a shoo-in for his work on the fundamental lemma (he’s a plenary speaker), and there’s some speculation about Jacob Lurie (who is a parallel session speaker).
This year there’s a new prize to be awarded at the ICM, the Chern Medal, for “an individual whose accomplishments warrant the highest level of recognition for outstanding achievements in the field of mathematics.” This one, unlike the Fields, comes with a significant amount of money ($250,000, and another $250,000 for the medalists favorite mathematical organization).