I got the urge to research this abit, now that time has passed, and to find more like minds that are still pondering on the origin of this elusive quote.
I've come up with these:
snopes.com: Strong Like Bull?
Google Answers: Strong, like bull (a good explanation is provided here, but has not been corroborated as of yet)
Both articles/questions come around to a semi-plausible answer that television actor
Hans Conried originally coined the term in film, though this is still debatable as some recall the phrase being uttered even before the mid-1950s.