Check to see if you have "Hide inactive icons" enabled in your taskbar properties. Might seem like a really stupid thing... but it is always a possibility. Just right click on an empty space on your taskbar and select properties.
It is possible that the icon has just slid into the inactive list. The inactive list is not really acurate... in order to stay on the active list, Windows actually expects that you normally click on the icon... otherwise it doesn't think you are using it.