Actually, I've seen this quite often where the chrckbox loads as 'null' and has been styled in a way such that the first click displays a checkmark, the second displays an X and so on, alternating from check to X and back. The main point is that the form validates that field on a non-null value so that the user is required to explicity click on the box. Keep in mind though, I haven't seen that use since the win95 days...Come to think of it, I remember a program called BitTerm(sp?) by Cheyenne software that used that in the initial config (I think to select whether your modem was Hayes-compatible or not, among other things)
I know most programmers out there will scream about how wrong a tri-state boolean is and blah-blah-blah. I agree. A true boolean should only be true or false. Therefore, while attempting to store three states from a checkbox into a two-state (boolean) variable is wrong and stupid, the existence if a three-state checkbox, if stored as another datatype (1 bit signed int comes to mind), is perfectly logical and does have a place, if used properly