[OS X Emacs] How to overwrite global line-wrapping setting for a specific mode?

[David Reitter]

On Oct 6, 2011, at 6:18 PM, Marius Hofert wrote:
> 
> With C-h k + the menu item "truncate", I found "set-truncate-lines", so I tried:
> (add-hook 'ess-mode-hook 'set-truncate-lines)
> Again, .R files were opened with word(s) wrap(ped).

This seems like the right solution to me, and it works perfectly for me, both with a manually entered R-mode, as well as when .R files are loaded.
It even says "Truncate long lines enabled" in the echo area after the mode is entered.

What's going on?