Atkins advises that you avoid it when starting off, can't remember the reason (I must re-read the book!) but I think it creates carb cravings, so best avoided if you can.
Having said that, I have a cup of builder's tea first thing in the morning and a soya latter - but don't have any more after 10am - and I hardly ever have diet coke now, when I used to have 2-3 cans a day.
Any more caffeine than that and I get heart palpitations!