There are many versions of CFs out there so look out.

1. The ballast is what controls the lamp and therefore the ballast must
be a dimmable type. The lamps are all the same. (Note that almost all
ballasts are electronic these days. Transformers for low voltage
lighting are still magnetic and are very different critters.)

2. Screw-in CF's have very small and usually cheap ballasts built into
them. There are a FEW that are designed to go on a standard dimmer. I
recall that Phillips made a big deal when they release some. As Phil
said: check the box! (and keep your receipts!)

3. Most commercial fixtures can be ordered with dimmable ballasts, but
they usually take special dimmers. They also work a whole lot better,
and more reliably....

So the short answer is; it depends on what your doing. However, a built
in ballast dimmable or not will always save money in the long run!

There is lots of other info on the web. Try:
http://www.eere.energy.gov/consumer/

Richard Reid, LC
Luminous Views
www.luminousviews.com