The "Mention Elvis" rule states: in pop cultural products that have nothing to do with Elvis but still mention his name, the less he is mentioned, the worse the product.
This law of diminishing returns hits its zero point when the product makes no mention of Elvis at all. Products that make no mention of Elvis are outside of the set of cultural predictions made by this rule... I will give some examples to demonstrate what I mean.
Take Bubba Ho-tep (Coscarelli 2002), which recycled the image of the King as part of a strange scenario starring Bruce Campbell. It's quite a good movie. Honeymoon in Vegas (Bergman 1992), with its flying troop of Elvis impersonators, references Elvis a bit less, and is an passable feature film too. Then yesterday I got to see Daybreakers (Spierig 2009), a new vampire movie in which Willem Dafoe plays a renegade rehumanized vampire, named Elvis, of course. The Dafoe character introduces himself as "Elvis, like the singer" and does a little impression. That moment, and pretty much the rest of the film, are so cringingly bad that even this talismanic mention of the King cannot save it. Elvis gets a tiny mention, and the movie is predictably awful, making lame comparisons between the American military-industrial complex and, um, blood suckers.
So why does the "Mention Elvis" rule hold up? I think it is because film makers using an Elvis theme for any length of time know it will wear very thin, very fast, unless they do something interesting. Meanwhile, scripts that merely reference Elvis are doing so to evoke a cliche that will connect with any audience. Everyone gets the joke. Nobody goes, "Who?" The result is that a stinker about Elvis can give the worst of material just a bit more milage.
Of course the "Mention Elvis" rule does not apply to Elvis's films themselves: neither the films about Elvis (which were usually good) or the films starring Elvis as a racing car driver, native indian or American playboy (which were usually bad). It also finds an exception in Ghostbusters (Reitman 1984) which cleverly used a mention of Elvis to highlight the stupidity of the media.