Quite often in my day to day work I find myself explaining the importance of headings for screenreader users as well as search engines. When building a CMS template I will use hn heading elements to represent headings in the proper way. However, once this is transferred to a CMS template, there's unfortunately nothing to stop the user using bold text to represent headings.
Apart from the fact that it introduces design inconsistencies across the site, it is bad for screenreader users and bad for SEO.
So, while searching for good ammunition to use in order to convince people of the importance of headings, I came across this excellent video by gringochapin:
http://www.youtube.com/watch?v=AmUPhEVWu_E
No amount of reading accessibility guidelines and theorising over best practices can compare to watching a screenreader user actually browsing sites (or even better - using one yourself). Developers and content editors - I urge you all to watch this video and take a good look at your document structure. You'll reap the benefits of good headings in SEO as well!
Web developer and all-round internet geek. Likes tea.