Hey fly,
An Ultrasonic tank is a liquid bath with a cleaner of some sort added in. On the bottom of the tank are one or more transducers(speakers) that blast sound waves at the part to be cleaned.
I have a little 2 gallon one that works just fine for my smaller parts. It seems to work particularly well on parts with complex geometries such as carb bodies. It also works well on cast iron. I think that is because the sonic wave can get into the pores in the cast and blast out the last remaining oil and other contaminates. It also works well on pot metal. I am not sure why that is. My first guess it that it blasts off any zinc plating or chromate that has loosened over time or as part of the cleaning prosses.
Here is a good introductory link
http://www.healthsonics.com/houke.htm
Dave