It depends on what is being focused on.
I've got nearly two and a half years' experience of the 40D's tendency to jump onto something other than the intended subject instantly if something else with more contrast than the subject encroaches even slightly into the area covered by the AF point - it happens, and is the price we pay for fast sensitive AF.
One of the great things about the 7D is that we can slow down the AF re-acquisition time to prevent this from happening.
But the acquisition time should just be for AI SERVO, not ONE SHOT. It is about the time you can lose the target before it changes the focus in case you acquire it again. ONE SHOT is just that, for focus on a single shot, not a series.
With my 40D, focusing on static objects works well in ONE SHOT but not so well in AI SERVO. With my 1D AI SERVO works surprisingly well even on static objects in poor light so I don't need to switch modes as much but with the 40D I definitely switch to ONE SHOT.
Focus points are certainly larger than they appear and may pick up the closer or more contrasty object if you aren't careful. Focus test targets must be chosen very carefully for this reason.