Still in beta, H.264 GPU decoding in Mac OSX is now officially enabled in the Flash Player. Here is some background info:

The primary reason this API exists is because we have been working with Apple to come up with a way to reduce power consumption on Macs. As we see more and more HD content on the web it is critical that we take advantage of hardware resources when available. In that context it is important to understand that this API targets HD content, not SD or smaller sized video. In fact SD sized content will not be accelerated in most cases. The decision of what content is accelerated and on which machine it is supported is up to Apple.