Half a day into my vacation, I managed to finish an article on a topic that has been intriguing me for a while.
Since several colleagues were baffled by the semantics of the new enhancements of MySQL 5.5 partitions, after talking at length with the creator and the author of the manual pages, I produced this article: A deep look at MySQL 5.5 partitioning enhancements.
UPDATE This matter was more tricky than it appeared at first sight. As Bug#49861 shows, several MySQL engineers were initially fooled by the multiple column partitions. Also I wrote something wrong in the article, and I updated the text to explain more accurately the behavior of the partitioning engine.