Does a pet CT scan replace a colonoscopy?

NO! Pet/ct is an excellent tool for cancer diagnosis and evaluation of therapy, but it does not actually 'look' at the lining of the colon. Often we see activity in the bowel that has been secreted, so it could mask or mimic a tumor.
No. Depending on what you are getting the pet scan for, they are more or less sensitive. If the pet is being done as surveillance or colon cancer, it is good for picking up recurrent disease, but will not pick up new polyps within the colon. You would still need the colonoscopy.
No... There is often diffuse physiologic tracer uptake in the bowel on pet/ct. More focal tracer uptake can often be seen in the ileocecal region in normal patients. Focal uptake on FDG pet often raises the suspicion for malignancy but may also be benign. Correlation with the ct portion of the scan is very helpful. Colonoscopy allows the advantage of biopsy or removal of suspicious lesions.