This is definitely true. AC is congruent to BD, and by the Isosceles Triangle Theorem, if this congruency is true, then the angles across from these congruent sides are also congruent. This means that angle ACD is congruent to angle BCD. That makes CD a bisector of angle ACB; it also makes CD a bisector of side AB. The definition of a perpendicular bisector is that the segment splits its angle in half (and it does) and that it splits the side it goes through in half (which it does). This makes CD, in a nutshell, the perpendicular bisector of AB (which also means that CD is perpendicular to AB).
