Diet sodas are acceptable, and you are allowed 3 caffeinated drinks per day. The new Momentum products have caffeine and are also good thermogenic products, but if you have them, you subtract the caffeine products allowed for each one you have.
Dr. Andersen mentioned on the Dr call wednesday that he would limit the diet sodas as they can do other things to us (the acids can affect our intestinal tract, the carbonation can bloat the tummy, and the acids also can wear away the teeth if you have too many). He also brought up that Splenda was tested extensively before it was approved, moreso than some of the other sweeteners.
So, yeah, 2-3 a week is fine... 1 a day wouldn't be the worst thing if you really need it