A long time ago, before shutter and mirror design was sufficiently advanced, Canon made some Pellicle mirror cameras. The whole point was that you'd have a fixed mirror in place that would reflect some of the light up at the viewfinder and let the rest through to the film.
The idea disappeared after a while, as shutters got better. After all, everything comes at a cost. Better to deal with the peculiarities of a swinging mirror and capture all of the light than dealing with a dim viewfinder and a 2/3 stop loss from the lens.
The notion of doing things differently than the SLR model now that most folks aren't shooting film has been frequently recurring. On the other hand, the mirror is not there just for style...
The electronic viewfinder camera has one major flaw... autofocus needs to use contrast-detection, not phase-detection... and phase-detection can be made to work faster and more accurately than contrast-detection. Thus, Sony is, instead of trying to use the mirror as a viewfinder, using the mirror just for contrast-detection autofocus.
I'm not sure how it will fare in the marketplace. The "SLT" setup is aiming offering a silly do-everything camera to advanced amateurs, letting you have more of the experience of a live-view SLR camera without some of the downsides. I suspect that, especially given that the SLR-styled Micro-4/3rds cameras that make no sense to me are still selling well, the amateur crowd will eat it up. But, honestly, I feel like the situations where the SLT setup excels in that the SLR wouldn't are so few and far between that it's silly to bother. Personally, I think the Fuji model of sneaking a phase-detection sensor into the main sensor is much more sound.
But you also have to understand that I'm the sort of person who hikes around with a massive Mamiya RB67 and a tiny Canon G7 and really doesn't understand why people make such a big deal about autofocus.
I'm still kinda looking for a better "small" digital. I'm still trying to wait out the small-interchangable-lens shakeout a bit more because the Micro 4/3rds, Sony NEX, and Samsung NX mounts are starting to get past the early adopter phase. I'm far too stingy to throw out perfectly good camera gear because it's obsolete.
Also, I'm kinda waiting for more of them to have 1080p video.