Scientists use language differently from the unwashed masses, they purposely avoid speaking in absolute term since nothing in science can ever be fully proven beyond doubt.
Creationists and intelligent-design boosters have a guerrilla tactic to undermine textbooks that don’t jibe with their beliefs. They slap a sticker on the cover that reads, EVOLUTION IS A THEORY, NOT A FACT, REGARDING THE ORIGIN OF LIVING THINGS.
This is the central argument of evolution deniers: Evolution is an unproven “theory.” For science-savvy people, this is an incredibly annoying ploy. While it’s true that scientists refer to evolution as a theory, in science the word theory means an explanation of how the world works that has stood up to repeated, rigorous testing. It’s hardly a term of disparagement.
But for most people, theory means a haphazard guess you’ve pulled out of your, uh, hat. It’s an insult, really, a glib way to dismiss a point of view: “Ah, well, that’s just your theory.” Scientists use theory in one specific way, the public another — and opponents of evolution have expertly exploited this disconnect.
This technique has been used science deniers in many areas, from evolution to climate change, and it represents a willful twisting of reality, not sound debate. When a scientific theory is dismissed outright because it is just a theory, it shows only one thing: an ignorance (willful or otherwise) of the scientific method.
UPDATE: The original article linked in this post mentions as a possible solution renaming long-standing rigorously tested theories as laws. Here is why this is a bad idea:
The theory of evolution is a whole collection of ideas describing complex phenomena; it is not reducible to the kind of clear and simple mathematical description we associate with scientific laws. When somebody asks me what the ideal gas law is, I can say PV=nRT; when someone asks me what the law describing the gravitational attraction between two bodies is, I say Gm1m2/R2; when they say, “OK, smartie pants, what is the law of evolution?”, what am I supposed to do? Recite Hardy-Weinberg at them (which, by the way, is called a law already, but is not the sum of all of evolution by any means)?
It’s a bad idea that sets us up for more confusion and will play right into creationist hands. Why not go all the way and just call it the “Truth of Evolution”? It’s the same strategy — it’s all avoiding the issue by an attempt at redefinition, and mangling the idea in the process.