Well, you're right that a dslr works different - however it's still
possible: just keep the light going to the sensor (so you won't be
able to use the viewfinder while shooting movies, dang), and
display what's being recorded on the tft screen.
Frequently discussed, but here's an approximate run-down.
Not if you're using a full-frame-transfer CCD. Possible if you're using an interline-transfer CCD, or a CMOS, if memory serves. The circuitry for live video takes space on the chip that might otherwise be used strictly for imaging so generally the CCD users have opted to maximize still image quality and forgo video.
It'll also heat up more and drain more power. More heat, more imaging noise, worse image quality, fewer shots before the battery drains.
It'll may also force you to do AF off the chip using contrast-detection (slow, as in most non-DSLRs) instead of the dedicated phase-detection systems normally used. At least if all light is going to the imaging sensor, which may not be necessarily true if a redesign is done.
Technically doable, 'tho, as long as one accepts those trade-offs. One could attempt to reduce certain of these trade-offs by adding complexity, e.g. a secondary imaging sensor and an additional mirror to allow one to switch between the OVF and the second sensor, which would receive the light that
isn't being sent to the AF sensors, as was discussed some time ago.
Of course digicam companies won't integrate new features like this
until they are forced to do so by the competition.
Bear in mind that for video, there's a cheaper route -- the dedicated digital video cameras. In light of that, it makes sense to optimize the high-end cameras for use as
still image cameras and sacrifice video.
