I've heard that the curtain cleaning can be done while they are still in place without having to remove them. Is this true?

The fact that curtains can be cleaned without removal would depend largely upon the method used by the cleaning contractor who should be able to advise accordingly; but to be perfectly honest from previous fitting experience I believe that removal and refitting would seem the better option.

Yes this is true, but the downside is that you can get upto 10% shrinkage and if they are lined, the lining could shrink at a differant rate to the face material. A good alternative is to hoover them with a curtain nozel and spay them with fabreeze

Buy a new pair...........less hassle.