I do use the pop up flash on the 7D to trigger a Speedlite and it works well, but I don't always want direct flash on someone's face, however I'm sure that a hot-shoe flash trigger for external flashes would be the answer for that problem.
What I really want to do, without spending a lot of money on a new flash -- I just want to try it -- is to photograph a moving object with a fairly slow shutter, with a strobe , creating in the photo, several instances of the same object. So yes, a rapidly pulsing light is the goal. From what I have read, the 580 will cause the 430, as a slave, to strobe. I was hoping that there would be a hot-shoe trigger that I could buy that would emulate the master/slave connection between a 580EX and a 430EX causing the 430EX to strobe. Sounds like I'm out of luck. Thanks for the replies.