I'm sorry that you've been waiting such a long time. Please understand that the Ask an Expert Forums are not chat rooms. We Experts check the forums on a rotating basis throughout the week. So, it isn't a good idea to bet on having an answer to your question the same day you ask it. In addition, repeatedly posting the same question doesn't help you question get answered any faster.
That being said, your question is an interesting one. I'm a planetary scientist, and NASA's Dawn mission is powered by an ion engine, which enables Dawn to make maneuvers that simply aren't possible with chemical propulsion systems. Ion engines open up all sorts of opportunities for doing great science.
As Keith said, an ion engine is powered by a plasma. It isn't powered by a solid, liquid, or gas. So, the question of whether an ion engine can use a solid, liquid, or gas as fuel is kind of like asking whether ice can power a water wheel. Just like ice can't power a water wheel, other states of matter can't power an ion engine. But, just like ice can be melted into liquid water, solids, liquids, and gases can go through phase changes to become plasmas, if you put enough energy into them. I'm familiar with two techniques that involve this: inductively-coupled plasma mass spectrometry and laser-induced breakdown spectroscopy. Check out these two articles to learn more about how those techniques work: http://crustal.usgs.gov/laboratories/icpms/intro.htmlhttp://en.wikipedia.org/wiki/Laser-indu ... ectroscopy
Now, just because we can turn a solid, liquid, or gas into a plasma doesn't mean that the resulting plasma will be useful for ion propulsion. The ion engine on NASA's Dawn spacecraft, for example, uses plasma made from xenon gas. Xenon is useful because it is a noble gas (it doesn't like to bond to other elements), it has a low first ionization energy, and a high charge to mass ratio. Those last two points make a xenon plasma quite efficient. There are other ion propulsion techniques that use liquid metals as the starting material (see, for example, http://en.wikipedia.org/wiki/Field_emis ... propulsion