I was diagnosed with it in my first pregnancy, but it was by an incompetent doctor. I think you are making a good choice. The other risk to consider is that if it is preterm labor rather than incompetent cervix that is causing your cervix to shorten, then you run the risk of your cervix trying to dilate with the cerclage in place and causing irreparable damage to your cervix. With my first, my OB didn't bother to monitor me for contractions and diagnosed me with incompetent cervix because I told her that I couldn't feel any contractions. It turned out that I was having them, I just couldn't feel them for some reason. Bedrest should do the trick, just make sure that you are really strict with it!! Good luck!
Amanda