Ed:
If a user does not have images turned on, it does not count as an open, even though they may have read it word for word in the text version. A text version can result in a click too, but not an open. As Jeff points out, even when images are turned on, different browsers can read it differently or it may may register as an Open even though they really didn't read it. Clicking a link is better measure of engagement.
Hope this helps.
Mike