More info: Using BLoglines as my aggregator (this also goes for Google, Yahoo): I find that changing the PERMALINKS format from Date & Name to any other forces the aggregators to get it right -- that is, this makes an update work. But it isn't the fix -- not even close! Once you have snagged the new material, further changes are not updated ... UNTIL you go an change the Permalinks settings again. Then it works ... ONCE. In other words, changing has an effect but the problem is exactly the same whetehr you are using the Default, Numeric or Date & Name.
I'm think the problem must be with the aggregator, not WordPress(???) but why would they ALL mess up in the same way? Also, this doe not correct after so many hours.
Perhaps WP only generates the feed XML once -- when you change the permalink setting?