I just posted a thread about this (I did search before creating, and didn't see anyone with my problem), but this post matches my symptoms.
I am having the exact same problem. My headset jack works for headphones, but when plugging in a headset, it treats it like headphones. the mic on the tablet is still working when the headset it plugged in.
---------- Post added at 12:46 PM ---------- Previous post was at 12:41 PM ----------
Based on these replies (one working unit is enough to prove it should work) and the number of people who say their external mic is not working, I believe that Google/Asus may have a mass defect in the units they have been shipping.
i think this as well, but the ones that are saying it is working, are they absolutely positive that the tablet mic isn't what's picking up their voice?
any easy test would be to do the skype test call, say "test 1" directly into the headset mic away from tablet, then "test 2" into the tablet mic. on the playback, you should be able to determine which mic is the one that is working since it should be much louder.
Another note... the little pamphlet guide that comes with the tablet describes the jack as a "headset" jack, not "headphone" jack. if it does not support a mic, that would be a typo.