More important to my thesis here: I notice what I notice.
My rhetorical edification (which can be attributed to formal education only in part) regarding vocabulary has embedded in my consciousness at least one thing: One would not go lay down; one would go lie down.
To lay is to put; that action requires an object: One could lay oneself down, but that's about as close as one can get with that phraseology.
Now, let's not overlook that when someone says "I'm going to go lay down" it is understood that the person is going to go lie down—to recline, presumably on a bed or couch or the ground—even by those of us who know the distinction. It's not a malapropism that utterly changes the intended meaning of the phrase. It indicates only that the speaker either doesn't know the difference between "lay" and "lie" or had a mental lapse at that moment, but even a listener who does know the difference is not left flummoxed about what was meant.
Ultimately, the lie/lay distinction is not that fair to quibble. Given that lie itself carries the multiple meaning of both "to recline" and "to tell an untruth" it's conceivable there are instances where someone wishes to distinguish that the meaning pertains to the former and saying lay might (in his/her mind) better achieve that without further explication. Also, given that when conjugating "to lie" there is a context where lay is actually used: for the past tense (such as "I lay on the bed last night, listening to the traffic outside"), so a conjugated version of the "right" verb is the "wrong" verb, so it's easy to see how keeping track of which is which could prove difficult.
I'm not suggesting those of us who have the distinction embedded in our brains need to start misusing the verbs in our own speech and casual writing; we can and should continue to demonstrate we know which is which, even with the full understanding that some people will think us erudite or condescending--or, worse, they'll think we're wrong when we use "lie" when they say "lay," but let's face it: If we were that concerned about what they think we wouldn't be in the camp we are; we've had to be our own self-generators of satisfaction regarding knowing something many others no longer consider important, so we should be well accustomed to this.
All I'm saying: Some perspective on the severity of the mistake is appropriate to observe.
Those of us who've learned the difference, rather than get upset when seeing or hearing an instance of that mistake, might be better served to channel that energy toward keeping an eye or ear out for those times when the speaker uses the correct verb for the context of the sentence and applauding that. Positive reinforcement and all that.
It can become like a secret handshake for an exclusive club. Rather than demand those who aren't really interested to comply with the tricky rules of usage, make it something where only those who make the effort are allowed in. Those of us who know the distinction between "lie" and "lay" could use that as something of a secret code that would allow us to identify one another. We'd turn the ostracism of being language geeks into an exclusive club. We would no longer harp on those who said "go lay down" but acknowledge those who said "go lie down." We'd know who we were and that would be good enough.
It's not that we'd kick out anyone who wanted in, of course. However, appealing to ostensible reason with those who don't care whether they're using "lay" when they mean "lie" is unlikely to be persuasive. We need to create a perceived situation where people want to be part of the "in the know" crowd (perhaps not entirely unlike the way a Mark Twain character got a fence painted).
What advocates of proper language usage may be better served to offer is seduction rather than admonition. Maybe replicating the same paradigm as the night clubs that have long lines outside, with implicit cachet associated with being inside by virtue of all those who are being kept out, is worth giving a shot. (I imagine a number of people reading that line who are among the intelligentsia bristle at the suggestion of mimicking the vapid environment of those who'd frequent such clubs, but I'd ask that those people analyze their reaction just a moment and ponder whether there's some element of sour grapes behind it; those clubs don't appeal to reason, but still people are waiting around to get in, so they must be appealing on a level that's effective.)
Of course, this error of verb choice is not uncommon, and has been happening for a long time. Eric Clapton's "Lay Down Sally" was a rather egregious example from back in the '70s; Snow Patrol's "Chasing Cars" (with the line "If I lay here, if I just lay here…") followed suit in the last decade.
Inside our club we'll have karaoke, but we'll distinguish ourselves by altering the lyrics when necessary:
Lie down, SallyUm, yeah.
Rest here in my arms
Don't you think you'd want someone with whom to speak?
Well, perhaps we need to make certain allowances for songs. Rock 'n roll ain't no term paper.
If one considers the "lay down" as contemporary slang that means "lie down," then the phrase is no more objectionable than any other word that's been appropriated for an alternative definition. And as slang goes, it's nowhere near as exclusionary as most of it tends to be.
The point of slang, of jargon, and the like, is twofold: It both allows for shorthand communication amongst those in the know, and it excludes those outside from grasping the meaning. It's essentially a code to be deciphered only by those who have made the effort to learn it.
So, yes, by that standard, the erroneous use of "lay" instead of "lie" is not slang; there's not the intent behind it to make it something one had to learn; it's frankly the opposite, something one failed to learn.
Okay, so let's pretend that the proper "lie down" is actually the slang (albeit slang that means exactly what it says). If we can pay attention to these distinctions in diction we can learn to delude ourselves that's the case.
What's the point of having developed one's intelligence if not to mold one's perceptions in order to make it seem like developing that intelligence was worthwhile?