Here are dates for the 2009 versions of some of the student robotic vehicle competitions:
International Aerial Robotics Competition (IARC) - The next event will be held during July, 2009. The venue will be the University of Puerto Rico at Mayagüez (UPRM) which is located in the city of Mayagüez, Puerto Rico. A 2009 Winner receives: $10,000
Seventh Annual Student Unmanned Systems (UAS) Competition - Naval Air - June 17 - June 21, 2009
The 17th Annual Intelligent Ground Vehicle Competition - Oakland University in Rochester, Michigan on June 5 - June 8, 2009
Student Autonomous Underwater Competition – Europe (SAUC-E) 2009 - Entrant forms complete by end of May 2009. The competition takes place in July 2009.
I don't know the information for the 2009 U.S. Autonomous Underwater Vehicle (AUV) Competition or the Autonomous Surface Vehicle (ASV) Competition.
The AUVSI Student Competitions site lists one I hadn't heard of before: 2008 UAV Challenge - Outback Rescue. It has an Australian high school component, and video documentary component, and an open search and rescue challenge that's not just open to Australian students, but also to anyone worldwide. The mission involves finding and assisting 'Outback Joe', a lost bush walker in Australia's vast outback using UAVs. It was held in September, with $70,000 in prizes available.