The only place i know that still gives discounts if paying by cash are chinese restaurants, mainly in north america. East to West of Canada and the USA, 90%+ of chinese restaurants i've been to will give you a discount if paying by cash.

Why just them, no idea... but a few more people will pay by cash if this option is avaiable (not the ones living off of credit cards, but the few of us out there that see an opportunity to save a little coin)... I know I would.