Diet soda is acceptable on the atkins diet. I don't know anything scientifically, but all the people doing no-day carb pre-contest diets eat splenda with their oatmeal in the morning if they can't shovel it down. Basically diet soda should just be a treat for yourself. Drink water, and research keeps telling me to not drink a lot while eating. I have been having diet pepsi and coke zero the past few days. I can't drink water 24/7. I only have a sip or two to feel refreshed, but I dropped significant weight in the past week. Most of it was water but that's still weight.
I would just ponder what I've written and not actually go by it since I have no scientific data telling me that diet soda isn't keeping me fat. More googling shows you should try to keep it under 3 packets of splenda/sweetener a day. It's not as bad as some other things that mess with insulin levels, but I'd say try and cut back as much as possible. It's only a few weeks of dieting.