In the US, several companies (such as GE) sell CFLs that are specifically
labelled as dimmable. This seems less related to the design of the circuitry
that starts the lamp, than it is to the presence of return path that allows
X10 and other signals to pass through the lamp.

I use mostly CFLs from Home Depot, which come on instantly, hit full
brightness in about 30 seconds, and have an excellent color balance. (These
are Consumer Reports' top-rated lamps, though I stumbled onto them -- there
was a nearby Home Depot, and they were cheap -- before CU reviewed them.)

These are not labelled as dimmable, but the X10 system will dim them. (There
are only about 8 or 9 steps from "max" before the lamp goes out.) There's a
catch, however.

If you use a conventional X10 plug-in wall dimmer to dim them, the lamp will
blink several times a second when you turn it off. The reason is that the
dimmer pulses the line periodically to determine when you've cycled the
power switch on the lamp itself. (This lets you turn on the lamp without
using the X10 controller.) The dimmer notes the off/on change in current,
and supplies full power. If you then command the dimmer to shut off the
lamp, it will continue to pulse the line.

According to X10, you need to use an X10 wall-switch dimmer, which doesn't
pulse the line. I haven't yet checked this.