In an attempt to reduce the numbers of outbound pings a a new undocumented "feature" was introduced in WordPress 2.8.
Instead of notifing ping services every time a post is published or edited, pings are now sent once every hour, at the most.
http://core.trac.wordpress.org/changeset/11410
I can't find anything about this feature in the release notes for 2.8, was the feature merged in by accident?
I think this change is really bad for the wordpress community and has discussed it on my blog:
http://www.karamell.net/2009/07/19/wordpress-putting-the-real-time-web-to-a-halt/