Canon DSLRs (or at least the ones I have used), when in Program, Aperture or Shutter modes will refuse to take the shot when the shutter speed would have to exceed the max speed. With auto focus on they will also refuse to take the shot if they have failed to get a focus lock. They will also refuse to take the shot in many modes if the flash is raised and the shutter speed needed would exceed the sync speed.

I'm guessing the Nikons and other DSLRs behave similarly ?

But of course is that is a problem you'd just switch to manual exposure and turn off auto focus.