I've not heard about the quinine one- our LLC told us that citric acid in diet drinks can disrupt fat metabolism, and bring you out of ketosis. Being a natural skeptic I investigated this and found that citric acid is part of the body's fat metabolism cycle, and so theoretically extra citric acid might interfere with it. I must confess however to have 'cheated' during abstinence by drinking Diet Coke (which has citric acid) and Coke Zero (which doesn't) without any ill effects on my weight loss. I have been convinced now that Diet Coke is not a 'good thing' for various other reasons and so should not be drinking it at all. Lots of other diet drinks contain fruit flavourings and other compounds (such as quinine) that may impact on the diet (although I'd like to see some proof about that).
I do wonder whether LL ban diet drinks so that they can sell their drink flavourings at £9 a go - if they let you drink Diet Coke etc they'd lose out on a chunk of their income...
Their is a lot of controversy about the use of Aspartame in diet drinks - I know Dr Atkins had a thing about this, but LL still use this in the St Clements flavouring- I think the have moved over to Sucralose (aka Splenda) in the Orange and Fruits of the Forest Flavouring.
While I wouldn't recommend cheating with diet drinks, I think that LL's ban on them for knocking you out of ketosis should be taken with a pinch of salt - however there seem to be good enough reasons to avoid them altogether.
The one that convinces me most is that artificial sweeteners of any kind can fool the body into thinking that sugar is on its way causing insulin to be produced, thus making you feel hungry and more likely to eat. But don't take my word for it!