Well, strange thing, there is new beta: http://labs.adobe.com/downloads/flashplayer11-2.html
with this one I have both accelerated decoding and rendering, but, also pretty high CPU usage ;/ - about 40% on one core (and about 2x50% while the video is buffering - is this an issue on windows?) Just like they said in the note, they can now use both (for me) cores, but not very well:
Unknown sequence number while processing queue
[xcb] Most likely this is a multi-threaded client and XInitThreads has not been called
[xcb] Aborting, sorry about that.
chromium-browser/chrome --type=plugin --plugin-path=/opt/Adobe/flash-player/plugin/libflashplayer.so --lang=pl --channel=14312.0x7f1c2727bd80.1807095856: /var/tmp/portage/x11-libs/libX11-1.4.4/work/libX11-1.4.4/src/xcb_io.c:273: poll_for_event: Assertion `!xcb_xlib_threads_sequence_lost' failed.
And overlay bug still exists.