I'm pretty sure the flash H.264 is either not optimised or not using hardware acceleration. The open bug on this was actually closed with a comment that it would only be resolved when an optimised codec was available (I'm sure I read somewhere flash 10 would support hardware acceleration of codecs, but I can't swear to it).

I think you're right, the stream though gstreamer should pick up the hardware accelerated H.264 codec and it should be grand.

If there is anything I can do to help let me know (I'm a software engineer with a background in video telephony, so this interests me!). Good luck getting something going.