I don't have my book handy, so I can't provide the exact quote - but in "Living the Low Carb Life", Jonny Bowden mentions there's a slight chance that the citric acid in diet colas can stall some people.
That said - there's plenty of people around here who drink lots and lots of Diet Coke, and no problem!
As for me, I gave it up. But, since I made a bunch of changes - I can't say for sure whether or not giving up Diet Coke made any kind of difference or not. Sorry..
And I don't think it was caffeine, because I still drink plenty of coffee.
If there's anything to this "Diet Coke stalls" idea - I'd say it's the citric acid, the carbonation or possibly the artificial sweetener. I've heard rumours that sometimes artificial sweeteners (Splenda or not) can possibly trigger more cravings for sweet stuff, potentially messing up one's adherence to plan and/or that artificial sweetener can affect one's blood sugar slightly, though obviously not to the degree that real sugar would. I don't have anything to substantiate these ideas, just things I've heard/read over the years.
Nowadays - I'll occasionally have some Pellegrino (mineral) water, but for the most part - no carbonated drinks at all. Water, iced tea - that sort of thing. But, for those times when I want that 'ahhhh' factor - Pellegrino with a wedge of lime fits that bill nicely.
Diet Coke may not be a problem at all - I'd suggest that you experiment. Go a few days without it, try having some - rinse and repeat, to see if there's anything to this.