But -and here's a question that I'm not really qualified to answer myself- would players whose last experience with the game was 1e, BD&D, 2e or something else recognize 4e as the same game given how far it goes to deviate itself mechanically from 3.x and from 1e/2e/3e AD&D in terms of flavor and basic world assumptions? (Of course would they have some of the same disconnect with even 3.x to a lesser degree?).
I don't know, either. However, I would suspect that if the real target audience is folks who haven't played in 15+ years, 4e's faithfulness to older mechanics wouldn't really factor much into that person's decision to get back into gaming. Such people likely were fairly casual players to begin with, and the mechanics are probably not of huge concern to them; the little bit of shared vocabulary should be enough to make it "D&D" for those who strayed long ago.
Another important thing about lapsed gamers: many of them are parents. I think the Essentials line-- and especially the "nostalgic" look-and-feel Red Box v4.0-- is likely being targetted at both the "lapsed" players
and their kids. I've got not idea how many folks around my age (36) remember the old Basic Set D&D red box, but I suspect that number is a pretty big chunk of the 24million quoted upthread. For that reason alone, I can see a LOT of Red Boxes showing up under Christmas trees this year and next.
With that in mind, I believe the exact nature of the system is largely irrelevant to the success of the marketing program: that first sale and the those first couple sessions with a parent and a couple kids is what really matters. If the 4e ruleset is compelling enough to keep a large fraction of those kids of lapsed gamers clamoring for more-- well, that's a success for WotC. More power to them.
As one who doesn't play 4e myself, I nevertheless think this is potentially a good thing. I would hope that some fraction of those lapsed gamers and their kids will find their inner DM via 4e and branch outward to investigate other RPG genres, the retro-clones & PF, indy games, and so on.