I actually found it, I think.
http://chandoo.org/wp/2008/10/01/simple-todo-list-using-excel/
It's just data validation > list > where your list is "Done, Not Yet"; the two possible values separated by commas.
For the sake of argument, this works for small lists that will never change. IMO, if it's a larger list or where the values might change or the list might grow/shrink, it's almost always preferable to create an additional worksheet with named range 'mylist', and point data validation to '=mylist'. Even with a rudimentary implementation such as this, 'mylist' can be adjusted manually to reflect new items.