There have been a number of studies about the effects of diet soda on people with diabetes:
Being overweight is a major risk factor for diabetes, and losing weight for people who are already diabetic is recommended. Excess weight leads to increased insulin resistance.
The above studies can only suggest that for people with diabetes, diet soda may not be a good choice. If you still choose to indulge once in a while, just remember that diet soda still has an impact on your health, so talk to your dietitian for advice on including it in your meal plan. The best drink choice of all? Just plain water!