In any event, accessibility is almost as poorly-known now as it was 2.5 years ago when I started work on my book. That's because most 'Web' developers aren't making Web sites at all, since they don't have a clue what valid HTML and CSS means.
Navigation is important, but the single biggest issue is prodding Web developers into learning how to make standards-compliant sites. They've quite simply been doing it wrong all along.