There are several sites that discuss this topic in detail.
Here are a few points:
Chein–
Although a fluorescent bulb by nature doesn’t incandesce (emit light and heat), the electrodes at the end of the bulb may glow red-hot, aside from the other problems on Shabbos.
Please see this thread for a much lengthier discussion on this and other electricity/ Shabbos issues:
http://www.theyeshivaworld.com/coffeeroom/topic/melacha-sheinah-tzricha-lgufa