All of this, disagreed. Mirror slap does indeed show an effect, in common conditions with a normal lens, even up to 1/125 and beyond. See Thornton 2000 pg65 for one published example... there are many to be found on the 'net of course. That said, proper holding or stabilization technique can mitigate though not eliminate the effect, and some bodies are more prone than others (bodies for formats larger than 35mm seem to suffer less, I'd imagine--but don't know--because of their weight).
Below 1/[lens focal length] it becomes necessary to use a tripod or set the camera down on a stable surface, some for sooner than others. The duration of mirror slap is so short due to dampening that a 10 second exposure is not going to show any effect, but it may be wise to lock up a mirror on long exposures as a good practice.