Another designer and I were having a discussion about the best way to show a list with checkmarks where only some of the selections have been chosen. This particular interaction will have multiple expanding/collapsing selections, each with the ability to select multiple items. His idea is to use something similar to what Gmail has with showing a dash for the sections that have only have some of the items selected. A section with none selected would have a blank checkbox. A section with all the items selected would have a fully darkened checkbox.
My opinion is that users will mistake that icon for a "collapse the section" icon and that something more like a grayed-out checkbox would work better to give the desired indication.
Have any of you run into something like this? Is the icon intuitive enough as it is or would something else work better?