Flash 10.1 supports hardware acceleration of H.264 on Windows.
From the release notes:
In Flash Player 10.1, H.264 hardware acceleration is not supported under Linux and Mac OS. Linux currently lacks a developed standard API that supports H.264 hardware video decoding, and Mac OS X does not expose access to the required APIs. We will continue to evaluate adding the feature to Linux and Mac OS in future releases.
I feel like this is a perfect microcosm of the respective worlds. Windows: getting shit done; Mac OS: would get shit done, but it's all locked down by Apple; Linux: nobody agrees on an implementation so we have none.