One thread blocks on select() and the other on dbus_connection_read_write_dispatch(). When ASUI is unmapped or the display is off the select has an infinite timeout and will only wake on events from X, dsme socket or the dbus thread message pipe.

I just finished playing the same song with mplayer for two hours straight, first hour with ASUI and second hour with systemUI, and a charge in between. The battery drained exactly 5.9% per hour with the screen off. ASUI doesn't seem to have any impact on battery life for me. Not sure of any other way to test this, none of ASUI's settings should change anything.

@ maacruz, do you know what non-zero values in /sys/devices/platform/dsp/state represent? When mplayer is active state=3 but the cpu clock is at 400 on the SSU kernel without op_dsp support.