The SEO Smackdown blog reports that they noticed Google switching "completely" to AJAX based search results. This can put a huge damper in rank checking tools, that scrap the Google search results. Most people are not seeing the Google search results pages render in AJAX, not yet at least.
But if this does happen, we can start hearing scandals of rank checking software being blocked by Google again. It turns out, what was blocking them was a change to how Google was testing new html for their search results page, which put these rank checkers out of business for about a month, until they came back in second week of September.
Google frequently tests different user interfaces and layouts with subsets of users. It is possible that this is just a test or it is possible that this is the being to a new AJAX based search result from Google. To the searcher, it makes very little difference, but to the SEO, it may matter.
Forum discussion at Sphinn.