The DAWN spacecraft crossed the inner limit of  the asteroid belt for the last time on November 13, 2009 on its way to the asteroids Ceres and Vesta.  The inner limit is defined as the greatest distance Mars travels from the Sun and is equal to 1.666 astronomical units.

The trip will be almost two more years longer arriving at Vesta in July 2011.

The video was done pre-launch and gives a pretty good explanation of the mission and you can find out more at the DAWN mission website.