No, not the British paperback of the 1980s penned by Eton pupils that raised and answered this question (a copy of which is floating around the disorganised library stacks of Coop Towers and probably would provide the basis of a good BritOSR post if I could ever find it) but a fairly serious question.
We know that Dave Arneson never actually played real D&D since his campaign rules continually evolved over the lifetime of his Blackmoor campaign and what first saw print under the title "Dungeons and Dragons" didn't much resemble the state of his rules at that time. And Dungeons & Dragons as a title was suggested by one of Gygax's daughters so presumably Arneson didn't even call his games that.
We also know that Gary Gygax stuck with an evolution of OD&D at the same time as pushing out AD&D with much nagging that this was the "real" version and we should all be using it.
I've long suspected that despite the desire of gamers to follow a set of RPG rules exactly as written, because the written word conveys gravitas and therefore must be correct, what we actually receive when we buy an RPG is a mixture of a snapshot of the ever-evolving game as it was played five minutes before deadline and a load of stuff thrown in because the authors see that the general public need rules for X,Y and Z and therefore they have to provide it. This is true no matter what the game and what title is slapped upon it. If the authors (Gary) don't play like this and still call their game "Whatever" why should players display a loyalty to "What's written there or the game isn't the game"?
So how much printed D&D do we need to use and still be playing "Dungeons and Dragons"? D&D is one of those terms like Biro and Hoover that has grown to cover the full spectrum of similar products but that's generally a non-gamer usage - if gamers say "D&D" they mean D&D. Not RuneQuest or Maid:The RPG or Cyberpunk etc.
Are we playing D&D if we use Swords & Wizardry or OSRIC or Castles and Crusades or Labyrinth Lord? After all, the rules of those games are published are almost certainly closer to the canonical printed versions of that game from TSR than most campaigns that used the original TSR game. Is it a purely semantic difference whereby the title is unimportant? Does S&W become D&D if I take a marker pen to the cover and amend the title? Can it never become real D&D because it's a different game whilst a long-standing game that has changed or misunderstood two dozen rules remains "real" D&D because once upon a time, it was intended to be such.
Life gets complicated when we consider the way that some games come with alternative rulesets. Call of Cthulhu currently exists in both BRP and d20 form and then there is Trail of Cthulhu using the radically different GUMSHOE system. Yet I feel that we are playing Call of Cthulhu using any of those three rulesets. Probably because the style and atmosphere and aims of play remain untouched.
A lot of games came out in the 90s with totally disposable rules because of the expectation that you'd simply throw away these poorly tested incomplete rules and use GURPS or Fudge. Are you then still playing the game you bought? Mash-ups are not something that just started with the internet age.
Are we playing D&D if we ended up porting the campaign over to GURPS or Savage Worlds? Is the WFRP Savage Worlds adaption that is floating around on the intertubes still WFRP? If a D&D campaign has been going for umpty-thousands years and very little of the original rules remain is that still D&D?
I think it is. I think it's akin to the axe that has had three new heads and four new handles. It remains the same axe.
D&D is D&D regardless of the rules so long as you self-identify that it's still D&D and still intend to regard it as such. It's the approach that matters. If you are playing with parties of ne'er do wells who seek treasure in dungeons and wildernesses it's D&D if you regard it as such. You might not regard it as such if you were doing the same campaign in WFRP. But perhaps if you'd ported a D&D campaign to WFRP because you prefered to rules, you might.
When Ken St. Andre saw an early copy of D&D he loved the idea and hated the implementation so went away and wrote a purely D6-based version. He called this "Dungeons and Dragons" because basically that was what he saw it as. Only when published did it change it's name to "Tunnels and Trolls". I like Tunnels and Trolls and call it by it's published title but if Ken continues to think of his game as being "Dungeons and Dragons", I'd fully understand where he comes from.
I think we are better off using D&D as a term for "feel and intentions" not rules.