Benchmark Reviews posted Adobe Flash HD-Video GPU Acceleration Guide. Here's a taster:

In Youtube, an easy way to force DXVA is adding &fmt=22 (for 720p) or &fmt=37 (for 1080p) at the end of the URL. This way you make sure the video resolution to be recognized by your Flash Player even if the video isn´t actually recorded at that resolution. Most problems will be solved on the official release and some will be solved with new ATI/NVIDIA drivers, but you can enjoy DXVA on many flash videos already. DXVA Flash Conclusion

I can resume the whole article in 5 words: DXVA works great on Flash. The reality is that both ATI and NVIDIA have done a great work on DXVA. Videos are reproduced flawlessly even on low-end systems, and CPU usage is lowered in a big percentage which sets your CPU free to do some other stuff at the same time. DXVA also allows HD flash videos to be reproduced on low end systems which won´t be able to reproduce HD flash with the CPU power, so that's a great start and the main target of DXVA.