That's a line in this recent decision from the Trademark Trial & Appeal Board. Technically it's dicta, but I think we can consider that important question settled. Next time I go to a ballgame I'm going to go up to the concession stand and say, "One hotdog sandwich, please," just to see what happens.
More importantly, the Board found that "footlong" is a generic term with respect to sandwiches, so Subway can't register a trademark on it.
Previous coverage of this issue here.
In case you're wondering, the last time I checked the litigation over whether Subway's "footlong" sandwiches are deceptively titled had been centralized into a Multi-District Litigation case in the Eastern District of Wisconsin. If I have time, I will see if there have been any post-worthy developments there.