the two most popular windows screen readers scrape the HTML from IE and produce a document outline based on <h1>, <h2>, etc. elements. WebKit should provide something similar for VoiceOver users.
VoiceOver has an ability to search by header. Control-Option-Command-h will go to the next header. Webkit doesn't need to do anything specifically. For VoiceOver specific bugs, please file bugs at http://developer.apple.com/bugreporter/ otherwise the VoiceOver team may not see them for years at a time