Yes, the Pentax's mirror does return automatically with a second slap, but that is not usually a problem :-). The first slap is just before opening the shutter.
Originally Posted by psvensson
As to the shutter induced vibration, when handholding I don't find it a problem, because this one goes INTO your hand as you're holding the camera. And your (or at least my) hand is quite good at dampening this. [in contrast to the mirror slap, which tries to move the camera up from your hands.] The shutter vibration is more of a problem on a tripod especially in portrait orientation when the camera is way off the axis of the tripod. I've found that with times of (1/30s) 1/15s to 1/2s it's better not to use a cable release, but hold the camera in both hands instead.
And yes, the sheer weight of the camera is enough to let you handhold what would be impossible/difficult with a feather-weight 35mm...