The folks who are arguing that the APS-C DSLR will always be around because there will always be uses for different size sensors are missing the point, I think. Of course there will always be optimal applications for different sizes of sensors. What doesn't follow logically is that the DSLR is an optimal format for incorporating smaller size sensors. In terms of image quality, Sony's mirrorless NEX 7 already competes favorably with any APS-C DSLR on the market. The same could be said for several new Panasonic and Olympus models--cameras which, on average, cost less and are much more comfortable to tote (if not to handle)--than an APS-C DSLR. Allowing for a few more generations of EVF development, and the last significant advantage of the APS-C DSLR will be nullified. Indeed, as another correspondent has pointed out, the small optical viewfinder in an APS-C DSLR is not that difficult a benchmark to beat.
The DSLR is an inherently heavy, bulky format. Two characteristics justify its existence given the current state of technology: (1) that big, bright, fast optical viewfinder and (2) its compatibility with the big, heavy, expensive but light-grabbing lenses that pro-level photography often requires. It's those lenses capable of generating a large, bright image that justify the bulk and expense of the modern DSLR. As infared asks, why throw that kind of optical firepower at a half-size sensor? And that's why I'm still betting that in ten years full frame will be close to a universal standard for DSLRs and a replacement for all but the most specialized MF systems. Smaller sensors will find a more natural home in other types of camera bodies. I think it's already happening. And I think Canon thinks so too. Look at where all the company's lens development resources are being directed. It ain't in EF-S, folks.