Flourescent dimming is done at the ballast, and uses its own special wall or computer control. The only flourescents on dimmers that I have worked with have been Lutron (and they are damned expensive!!).
I believe they cut the current, but keep the filaments hot so the arc stays on, even when turned down low. Not sure about the frequency or voltage. They definitely stay the same color (to my eye anyway), but simply dim down like halogens.
http://www.lutron.com/CMS400/WorkAre....aspx?id=10055