AFAIK the Magnuson-Moss Act is still in effect and they have to prove that your modification caused the problem in question. Problem is you would have to take them to court to get them to do it and you would spend more on an attorney than it's worth most of the time, which is why they get away with voiding warranties so often.
I believe they can plug in and tell on any of the Bosch ECMs if they have been tuned, even so the above paragraph applies. If you correct your speedo yourself with EFI they "can't" void your warranty for work on say injectors, CP3, turbo, ect. Whether they make you fight them in court is another story.