I managed to get the subwoofer on my Inspiron 9400 working today. I had read in several places that I just need to add 'model=ref' to the module config file, but I'd tried that without success.
Today I discovered that openSUSE 10.2 uses version 1.0.12 rc2 of the alsa drivers, and they don't support the subwoofer. So I downloaded alsa-driver-1.0.13, configured it with ./configure --with-cards=hda-intel, ran make, and then make install-modules, rebooted my machine, and the sub was working. Oh and that model=ref line was added to /etc/modprobe.d/sound, so now the first line is options snd-hda-intel enable=1 index=0 model=ref.
There's only one slight problem - the master volume control doesn't adjust the subwoofer (shown as LFE in the mixer), and muting the sound also doesn't mute the sub. But other than that it works!