Pippaki,
Lots of people drink Diet Coke or other diet sodas on this diet without problems. Many people choose to avoid sodas with aspartame (almost all diet sodas are sweetened with this) because of 1. the caffiene and 2. aspartame is said to stall a lot of people.

It's totally up to you. I drink 1 Diet Rite (a diet soda sweetened with Splenda) a day, except for maybe 2 days a week when I'll have a Diet Coke.