As Matt points out, auto focus systems require enough contrast in order to detect the image of desired focus. Two limitations do exist, however. One is the distance of effectiveness, as Jeff points out. The other is that with projected grid lines, what you really want in focus may not be in the same focal range as items in the grid that your auto focus picks up on.
There are now accessories you can buy to assist with focus that provide a single point of light (rather than a grid) and have an effective range of over 100 feet. With those you can get the fast focus you want, on exactly the subject you want, and do so throughout the effective focal range of your lens. They use the same principle as your speedlight - providing that point of contrast for the auto focus to pick up on.