There is currently a discussion on www.Wikipedia.org about whether human factors and ergonomics should be merged into one topic. There is quite a difference of opinion. I think this is partly because the terms are used differently in different countries and settings.
The main question seems to be whether ergonomics is more focused on anthropometrics and human factors on cognitive and perceptual factors. Those who support the merge seem to suggest that both terms cover the whole subject. Those that oppose seem to suggest that there is a big overlap, but the different focus constitutes separate subjects.
To see the debate go to http://en.wikipedia.org/wiki/Talk:Human_factors
For those of you who don't know, Wikipedia is an online encyclopedia that is made up through informal collaboration. The idea being that anyone can add or edit, and over time the subjects will evolve to a consensus.