Using pulseaudio in combination with alsa is already more power efficient and avoids many interrupts on current systems. In my experience the interrupts created by hda_intel decreased from 60-80 down to 6 per second. The sleep residency increased from about 3 ms to about 8 ms. This already makes a difference of about .25 Watts. I wonder if and how the difference will be with the new patches that went into 2.6.38, disabling even more interrupts.
Update: Also check how to save 0.4 Watts and more with pulseaudio and alsa.