Thanks everyone from the feedback (and lshal entries) from my last blog entry.
I have got lots of data on this, and am now in the process of getting a couple of patches finished and (hopefully) approved to be added to HAL sometime soon.
I've added initial code to gnome-power-manager (it's not going to work yet!) for the warning notification. I asked my girlfriend (a typical non-technical user) if she would check her battery faced with that warning and she said probably not as it didn't seem very serious.
I'm not sure I can (from a legal perspective and user sanity point of view) put up a window saying “YOUR BATTERY MAY EXPLODE ANY SECOND!!!” but the wording could do with some work. Comments please.