My car is 4 years old with bolt on mods, and it has been tuned for close to 4 years. I've always been completely upfront with my dealership about my mods, and I've never been denied any warranty work. I just recently had several parts replaced under my power train warranty.
So you can't just make a blanket statement that your warranty is void if you get your car tuned. It a case by case situation and it all depends on the part/parts in question.