"15 items or less"? It's OK, Al, really.
This video from Weird Al Yankovic is making the rounds and being praised in places like the Linked In Editors and Writers forum.
But you know what, Al's poking fun, methinks, not at the less/fewer conundrum but at those who would get all upset over it because you know what -- "xx items or less" is not wrong.
In this case, the phrase serves as an inflection point, really a binary condition, not a count situation. Listen to Wendalynn Nichols' podcast on this ill-advised rule (go to Stupid Rules 11 of Oct. 12). Or you can read it at her Copyediting tip of the week.
There is a lengthy discussion there of "continuous" vs. "discrete," but I side with Nichols on this. Garner, on the other hand, refers to the "linguistic hegemony by which less has encroached on fewer's territory" and concludes it is, indeed, the result of the checkout line kerfuffle, though he notes that "the occasional more literate supermarket owner uses a different sign" with fewer.
But then, in regard to percentages, he goes on to advocate "less," advancing the idea akin to continuity - that most percentages aren't whole numbers anyhow - and concludes: "And even if it were a toss-up between the two theories, it's sound to choose less, which is less formal in tone than fewer." I'd say the same idiomatic argument can be made at the checkout.
Now, let's open another sore point: Plastic or paper ...?