I've had this happen too. It was simply from an app not having a supported/any notification bar icon. It was also quite simple to figure out which app it was by pulling down your notification area & comparing.
Some apps just don't have notification bar icons like the toggle widget above, other like mine do(mine is only setup to show status at this time but toggles can be added-it also has a choices of which icons to use). So if you have a blank icon in your notification bar, you either have something in the notification area that has no icon, no icon set, or maybe no supported ICS at this time(I think I had one of these when I first updated-maybe I was just no icon set though).
Here's a couple of screen shots, one showing no blank icons & one showing a blank icon(I switched the icon for tasker to non).