HD decoding on these boards is performed using a digital signal processor (DSP) integrated into the SOC, the CPU (even with GPU support) is too slow for 1080P h264 Content. This is not specific to Android, for example TI supports use of the DSP of the Pandaboard on Linux as well [2] (it is still not very stable, at least on the OMAP 4430 version of the Pandaboard).
[1] http://airlied.livejournal.com/76383.html
[2] https://launchpad.net/~tiomap-dev/+archive/release
Edit: formatting
I know this works on pandaboard and some others, but those boards are about 150$ range if i remember correctly.
There is an unofficial XBMC port for A1X devices which supports the HW Video Decoder of the SOC [2]. I have an A10 device at work, but unfortunately won't be in the office to play around with it for some time.
According to what XBMC devs say - it doesnt work at this point, and there is some online drama going on ;-)
Although i would be super happy to see progress on this.
One of the links you gave me: "Allwinner officially only supports the Mali GPU driver on the Android platform only." :(