In Cabel's blog post about this prank, he explains the importance of being subtle with sign pranks by writing, "The comedy threshold is very delicate. The goal is to fall somewhere in the realm of extremely general believability. If you do something outright offensive ("Poop Soup"), or something too obviously insane ("Horn of Unicorn" or "Children Con Carne"), you'll only accomplish one thing: the next customer will look, laugh, ask, and the sign will be taken down immediately. You want big burly men to take a look at the sign, ponder it for quite some time, and apparently seriously consider ordering the Bison Noodle Soup before having second thoughts. The game here is to pick something funny, but not so funny as to elicit questions."
Exactly my reasoning for the subtle wording of the first McDonald's sign I put up. I'm surprised his Bison Noodle sign didn't stay up longer since it doesn't seem so far-fetched that such a thing wouldn't exist. I mean, they have bison burgers, don't they? But the cool thing about Cabel's prank is that he made several versions of this sign (one of the flavors is Cream of Regret) and put the templates for them all on his blog for other people to print and use at their local Subways.
Visit his blog to read the whole story, see the original sign, view his other versions and download them to use yourself. Be sure to check out the comments too, since there's some funny Subway sign ideas in there.