I've come across the same problem with a game I was writing recently too. On my desktop, I had no problems, but on my laptop, I was unable to capture left+down+space. If you are using the arrow keys for movement and need a third action key, then the only real solution I know of is to have the option of using the mouse button instead of the spacebar. It's a pain, though, as the mouse is far from ideal in Flash games because it doesn't register outside of the Flash window. I would love to know if anyone has an elegant solution to this. Perhaps we should create a little key-combination tester in Flash and conduct a survey, to see if we can identify any combinations of keys that always work for everyone.