Found it!
"I've got an idea for a story: Gus and Ethel live on Long Island, on the North Shore. He works sixteen hours a day writing fiction. Ethel never goes out, never does anything except fix Gus sandwiches, and in the end she becomes a nympho-lesbo-killer-whore. Here's your sandwich."
For years I have searched to find this cartoon again — and now, of course, this being the Internet age, it's just a matter of knowing where to start looking: in this case, at cartoonbank.com. You will find the cartoon in all its glory right here, surrounded by a bevy of merchandising offers. For example, for only $24.95, you can get the cartoon emblazoned on a t-shirt...
I actually looked into licensing the cartoon for use on a web site, only to learn that licensing fees start at $250/month and go up from there — which, I daresay, is probably more than cartoonist George Booth was paid for the original, back when it was first published in 1970. Here's hoping that Mr. Booth, who is still alive, is getting a generous share of whatever revenues his work might be generating for the Advance Publications empire, but in the meantime, let this be a lesson to you. It's not about being first, or fresh, or original, or creative; it's about controlling the rights.