I posted regarding this issue to alsa-users, but haven't had any response. I've come to the conclusion that I will have to modify the driver myself to rectify this problem, so I would appreciate some pointers as to how to get started with that.
Running Xubuntu 10.10 with Alsa 1.0.23
The sound chip is Intel HDA, ALC260. I have tried setting
options snd-hda-intel model=fujitsu
in /etc/modprobe.d/alsa-base.conf, but this hasn't made any difference. There is no "Jack Sense" option in alsamixer. I've also tried a couple of values for fix_position.
I've tried following the ubuntu wiki pages for Intel HDA and sound debugging in general, to no avail. When doing the jack sense test recommended on https://wiki.ubuntu.com/ALSA/JackSense the two files generated with and without the headphones plugged in were identical. This makes me think it is a driver problem. I pasted the output of `cat /proc/asound/card*/codec*` at http://pastebin.com/4NyvdsXf in case this is of any help.