I just realized a really annoying problem using Ajax to hide content. An obvious example would be if you have a set of Ajax tabs that you use to show different content in a set space, as seen in several jQuery example sites.
This results in a thoroughly infuriating experience of me clicking on random things in order to try and get my desired content appear. NOT what we want.
I have written a follow-up post with a possible solution here:
smartTabs :: SEO friendly AJAX tabs