This change was confirmed in a tweet from Matt Cutts. His comment ("Googlebot keeps getting smarter. Now has the ability to execute AJAX/JS to index some dynamic comments.") gives an indicator of the limitations on the update. It seems that the changes were built for a specific sort of content; not all comments built in AJAX/JS will be included, and it's not a universal change that allows all dynamic content to be indexed.
In the meantime, webmasters can approach dynamic commenting options with less concern for the potential SEO consequences, and users will gain access to the large amount of content posted directly to these AJAX/JS-based mediums.
It's not yet certain what other SEO or user impact this will have; the possibility of link-juice being passed from these comments, the comments leading to an increase in social search rankings, or the inclusion of these posts in real-time search have yet to be tested or explored.