By law a modification cannot void your warranty unless it can be shown to have caused the specific, warranteed failure. Even then, it cannot void any other, unrelated part of the warranty. Dealers that spread this gossip are not to be trusted. I understand their potential liability, and they can choose whether or not to install a mod, but they should explain that they are not comfortable with it, or don't want to be bothered, not that it affects customer warranties.
-Scotty