An Associated Press article is reporting that NASA has postponed the launch of Dawn indefinitely.
Dawn is an ambitious mission to visit two of the largest asteroids in the solar systems, Ceres and Vesta. It has an ion engine, which doesn't use combustible chemicals for propulsion, but instead uses a powerfulmagnetic electrostatic field to fling individual atoms out the back at high speed. Although the thrust is low, it can be sustained for months, even years. It's a tremendously more efficient way of propelling unmanned probes around the solar system.
Dawn has had a checkered history, with several mechanical and funding problems. I don't know why NASA has postponed launch (the AP article vaguely states "cost overruns and technical issues"), as I just heard about this from Larry Kellogg's lunar-update mailing list. When I get more information I'll post again.
Note (Jan 24, 2006): I originally said the ion propulsion used magnetic fields to accelerate ions, but this is incorrect. A reader informed me that it's a strong electrostatic field that does the trick.'