I am using the Standard client on OS X 10.5.3.
I always use the preview pane to read mail. In the inbox, all is fine, when I click on a message it shows as being read and the unread count goes down. In any other folder, clicking on the message does not show it as unread, and the count does not go down, but if I then click on another folder, the unread count and the unread status of messages in the previous folder gets updated. This did not happen in the previous Mac-only beta, that version worked fine.
I live in the Preview pane, so this behaviour is very annoying.