I replaced the incandescent bulbs in a 6 bulb fixture with
non-dimmable CFLs. I didn't realize it at the time I did it that the
switch was a dimmer switch. Realizing it was a dimmer switched
circuit I tried dimming them. Didn't work, they were either on or
off. So just to see what would happen I took out two of the CFLs and
put regular incandescent back in. With the inc bulbs there the dimmer
would dim the non-dimmable CFLs. Not as well as real dimmable CFLs
but certainly enough to be dim enough to satisfy my needs in the
dimming department. there is a slight buzzing when I dim this setup
but it's not enough to be a problem to me for the few times I want to
dim them.

So what's different about the dimmable CFLs compared to the
non-dimmable ones? Do they just add a resistor in parallel inside the
dimmible bulbs???