The toolbar button that toggles the todaypane is supposed to be located left of the first separator. Under Macintosh this button is placed at the very right end of the toolbar. I assume that the reason is that Macintosh uses spaces to group toolbar buttons and not separators like the other operating systems. The implementation of the today-pane gears to the first existing separator and if this is not available it places the toolbar button at the right end.
This issue became obsolete by the checkin of Bug 456362.