The NASA Dawn mission is approaching Vesta and should enter into obit tonight. We will know if it was successful tomorrow morning. It will stay in orbit around the asteroid for a year before gently pulling away using its ion drives to head out for Ceres which is now classified as a dwarf planet.
Studies of Vesta and Ceres could provide clues about the early formation of the solar system.
Update 17-Jul-2011: It has been confirmed that Dawn successfully entered orbit around Vesta.
Update 20-Jul-2011: This new picture now available