I think Diet Coke contains the artificial sweetener Phenylalanine.
If you do some research into it you will find that there is a fair bit of controversay on the safety of phenylalanine.
Everything you read needs to be taken with a grain of salt however.

I have decided that if I get pg I will not be having more than glass/can of diet drink or one low fat/low sugar yoghurt per day.

And I would try to keep the caffeine to a minimum too...

But thats just me... I like to err on the side of caution.