Hm, have tested just now with my ALC882, Mic-In is working fine (stereo!)... have recorded and can play the recording fine.
Here it is working...
(You can try to use your iPod-Headphone as an Mic, must set up the recording volume to max...)
The asus P5w DH has the alc882M codec as opposed to yours which has the alc882. i wonder what the difference is between the 2.