Some things--the iPad comes to mind--are clearly so overwhelmingly accepted and adopted by users that they are a clear and definite trend that predicts the future of a whole group of related products. Other things show different adoption curves, ones that aren't straight lines up and to the right on the inevitable graphs.
Interchangeable lens camera growth has been modest the past two years. But take out mirrorless cameras, and DSLR growth has been negative. (It's difficult to verify that last point because there aren't any easy statistics to back that up--you have to do a lot of careful assembly of multiple sub-statistics to come to that conclusion, but I believe it to be accurate).
The question in my mind is how much of the mirrorless purchasing is "sampling" versus "adopting." It's clear from my surveys that there's a backlash on big, heavy, feature-laden DSLRs. Sure, the pros still love them, but the pros don't drive the mass market. More amateurs buy D3 series bodies than pros, and after doing so, one of the most common comments they have is "love the image quality, don't like the size and weight." That's true even for smaller DSLRs, such as the D300s and D700. Given that airlines are likely to start charging for carry-on weight soon, I can only see the size/weight complaints increasing.
Still, the complaint is already there and in significant volume. Every day I receive many "I'd love the same quality, only smaller" emails in my In Box. These are usually accompanied by the "is mirrorless good enough" question.
As should have been clear back in summer of 2009 when I first started experimenting with mirrorless cameras, I started out a "sampler" or mirrorless, not an "adopter." I brought an E-P1 to Africa and managed to take some very nice pictures with it. My sampling, however, wasn't to replace my DSLRs, but rather to replace compact cameras (e.g. Canon G10 at the time).
Since then, I've pretty much always had a mirrorless camera with me, as I did go from sampler to adapter. But again, as a compact camera replacement, not a DSLR replacement. Simply put: a GF2, E-PL2, NX100, or NEX-5 make for a very competent compact camera replacement, and one that has a lot of flexibility. Yet they don't yet really make it to DSLR replacement in my mind, though the NEX-5 is basically there in image quality.
I'd say that the primary mirrorless user has been a "sampler" up to this point. People are testing two hypotheses: (1) whether something slightly bigger than their compact camera gives them more performance and flexibility; and (2) whether something much smaller than their DSLR gives them enough performance without sacrificing too much other capability.
The camera makers see things differently. They see that a couple of million mirrorless units have been sold. If you're not playing in a market that sells in the millions a year, you ultimately get hurt. So we'll see Nikon, Canon, and Pentax all come into the mirrorless market, too, while Panasonic, Sony, Samsung, and Olympus redouble their efforts. These camera companies aren't sure why mirrorless cameras are selling, but that won't stop them from entering the market or further pushing their efforts. Go back and read what I wrote about Olympus original misguided marketing for the E-P1. At least they've figured out that that wasn't their target market, but I'm not sure they've figured out what is.
But consider this: a D700 is ~83ci and 35 ounces while an FM2n was ~47ci and 19 ounces. Cameras don't have to be as big and bulky as they are. As one reader wrote me: "I just don't get it. I mean what is wrong with Nikon? They were able to make that excellent FM2 and now we don't have anything that fills its place."
To me, the camera companies are chasing the wrong rabbit. What users want is smaller and lighter, not "remove the mirror box, optical viewfinder, and sophisticated focus system."